Section 54. It Is in the Demonstration of This Wonderful Unity in Life,

only the more confirmed the more exhaustive our analysis becomes, that the educational value and human interest of biology chiefly lies. In the place of disconnected species of animals, arbitrarily created, and a belief in the settled inexplicable, the student finds an enlightening realization of uniform and active causes beneath an apparent diversity. And the world is not made and dead like a cardboard model or a child's toy, but a living equilibrium; and every day and every hour, every living thing is being weighed in the balance and found sufficient or wanting.

Our little book is the merest beginning in zoology; we have stated one or two groups of facts and made one or two suggestions. The great things of the science of Darwin, Huxley, Wallace, and Balfour remain mainly untold. In the book of nature there are written, for instance, the triumphs of survival, the tragedy of death and extinction, the tragi-comedy of degradation and inheritance, the gruesome lesson of parasitism, and the political satire of colonial organisms. Zoology is, indeed, a philosophy and a literature to those who can read its symbols. In the contemplation of beauty of form and of mechanical beauty, and in the intellectual delight of tracing and elucidating relationships and criticising appearances, there is also for many a great reward in zoological study. With an increasing knowledge of the facts of the form of life, there gradually appears to the student the realization of an entire unity shaped out by their countless, and often beautiful, diversity. And at last, in the place of the manifoldness of a fair or a marine store, the student of science perceives the infinite variety of one consistent and comprehensive Being-- a realization to which no other study leads him at present so surely.

To the student who feels inclined to amplify this brief outline of Vertebrate Anatomy, we may mention the following books: Wiedersheim's and Parker's Vertebrates, Huxley's Anatomy of the Vertebrata, Flower's Osteology of the Mammalia, Wallace's Distribution, Nicholson and Lyddeker's Palaeontology (Volume 2), the summaries in Rolleston's Forms of Animal Life (where a bibliography will be found), and Balfour's Embryology. But reading without practical work is a dull and unprofitable method of study.

Questions on Embryology

1. Describe the changes in the egg-cell which precede fertilization; describe the process of fertilization and the formation of the primary cell-layers, as exhibited, in three of the animal types known to you. What is the notochord, and how is it developed in the frog?

2. Describe the early stages in the development of the egg of the fowl as far as the closure of the neural groove. How do you account for the primitive streak?

3. Describe the cleavage and the surface appearances of the egg of the frog and of the rabbit, up to the time when the first gill-slits appear in the embryo. Give illustrative diagrams of what you describe.

4. Describe the structure and cleavage of the ovum (a) of the frog, (b) of the fowl, and (c) of the rabbit. (d) Explain as far as possible the differences in the cleavage of these three eggs. (e) Point out how the embryo is nourished in each case, and (f) describe the constitution of the placenta in the rabbit.

5. (a) What are the protovertebrae? (b) How does the notochord originate in the frog? (c) How are the vertebrae laid down in the tadpole? (d) Describe the vertebral column of the adult frog. (e) In what important respects do the centra of the vertebrae of the frog, the dog-fish, and the rabbit differ from one another?

6. Give an account of the more important features in the development of the frog.

7. What temporary organs are developed in the embryo frog which are absent from the embryo bird and mammal, and what in the two latter which are absent from the former?

8. Draw diagrams, with the parts named, of the heart and great arteries of the frog, giving descriptions only in so far as is necessary to explain your diagrams; trace the development of these structures in the tadpole; point out particularly in which of the embryonic visceral (branchial) arches the great arteries of the adult run.

9. Trace the history of the post-oral gill-slits and their accompanying cartilaginous bars and vascular arches in the frog, fowl, and rabbit.

10. Give a short account, with illustrative figures, of the mode of formation of the primary germinal layers in amphioxus and in the frog. What explanation can you give of the differences between the two cases?

11. Give a short account, with diagrammatic figures, of the principal changes which occur in the circulatory and respiratory organs during the metamorphosis of the tadpole into the frog.

12. How do protozoa differ from higher animals (metazoa) as regards (a) structure, (b) reproduction? Compare the process of fission in an amoeba with the segmentation of the ovum in amphioxus, pointing out the resemblances and differences between the two cases.

-Miscellaneous Questions_

1. Describe (a) the digestive, (b) the circulatory, (c) the excretory, and (d) the reproductive organs of the amphioxus.

2. Describe the stomach and intestines of the dog-fish and rabbit, and point out in what way their differences are connected with diet.

3. Describe the mechanism of respiration in the adult frog, and contrast it with that of the tadpole.

4. Give an account of the structure of the epidermis and its outgrowths in the frog and the rabbit.

5. Describe the organs of circulation (heart and main arteries and veins) and respiration in the frog in its mature and immature states.

6. Give a brief account of the physiology of respiration. Describe fully the means by which respiration is effected in the following animals:-- frog, amphioxus, rabbit, and dog-fish.

7. Describe the minute structure of the blood of the rabbit, frog, and amphioxus.

8. Describe and illustrate by means of sketches the chief points of difference between the skeleton of the rabbit as a typical mammal, and that of the common frog as a typical amphibian.

9. (a) Explain what is meant by the term "central nervous system." (b) Describe the tissue elements which enter into its composition. (c) Explain, as far as you can, the function of each structure described. (d) How is the central nervous system developed in the frog, and (e) in the rabbit? (f) What conclusions may be drawn from the facts stated as to the origin of the central nervous system in evolution?

10. Give an account of the structure (including histology) and of the functions of the spinal cord and spinal nerves of the frog.

11. Give a description of the minute structure and chemical characters of the following tissues as seen in the frog:-- cartilage, bone, muscle. From which of the primary cell-layers of the embryo are they respectively developed?

12. What substance is excreted by the renal organ of a frog, and what relation does this substance bear to the general life of the organism? Describe the parts by which similar excretion is believed to be effected in amoeba, hydra, earthworm, mussel, and lobster.

13. Describe, with illustrative sketches, the structure of the connective tissue, cartilage, and muscular tissue of a frog. Also describe the structure of the muscular tissue of the lobster and snail.

14. Give in account of the more important features in the development of the frog.

15. Describe and compare the structure of the renal organs in a frog and a rabbit.

16. Give an account of the structure of the genito-urinary organs of the frog. Compare these organs of the frog with those of the dog-fish and of the rabbit. Distinguish in each case the conditions of the two sexes, and describe briefly the microscopic structure and development of the ova and of the spermatozoa.

17. Describe, with diagrams, the arrangement of the urinary and generative organs in the male of (a) the rabbit, (b) the dog-fish, and (c) the frog; (d) point out the most important differences between them.

18. (a) Describe the structure of the ovarian egg of the rabbit, (b) and of the pigeon, (c) and of the frog; (d) from what part of the embryo do they originate? (e) What is the structure and origin of the ovarian follicle in the rabbit, and (f) of the ovarian stroma? (g) What is the "granulosa" and what the "zona pellucida"?

19. Describe the pre-segmentation changes, mode of impregnation, and early stages of development in the ovum of the frog, as far as the closure of the neural canal.

20. Illustrate, with diagrams, from the structure of typical organisms, the principle of repetition of similar parts.

-Note on Making Comparisons_

Students preparing for examinations are frequently troubled by "comparison" questions. Tabulation is often recommended, but we are inclined to favour a rather more flexible plan of marking off differences and resemblances. In tabulation a considerable loss of time is occasioned by writing down the features of both the things compared, and this is a serious consideration for the examinee. We advise him therefore, first, if he possibly can, to draw side by side and in corresponding positions the two things under consideration, and then, going over them in a methodical way, to state simply the difference between each homologous part. We append as examples three test answers actually submitted (with figures) in "Correspondence" work:--

1. Compare the brain of the frog with that of the rabbit.

In the frog's fore-brain--

The olfactory lobes are fused in the middle line.

There is no corpus callosum, nor is there a middle commissure to the third ventricle.

The cerebral hemispheres are not convoluted, and, looked at from the dorsal aspect, do not hide the thalamencephalon and mid-brain.

The pineal gland lies in the cranial wall and not deeply between the hemispheres, and its stalk is longer and tilts forward.

In the mid-brain--

The optic lobes are two, instead of being corpora quadrigemina, and hollow.

In the hind-brain--

The cerebellum is a very small transverse band, and has no lateral parts.

The medulla is relatively larger.

There are no spinal accessory nor hypoglossal nerves to the brain.

2. Compare the vertebrae of dog-fish, rabbit, and frog.

The centra of the dog-fish are -opistho- [amphi]-coelous (i.e., hollow at either end).

The centra of the rabbit are flat-faced.

The centra of the frog are procoelous (hollow in front).

The notochord persists between the centra in the dog-fish and rabbit, within the centra in frog.

The centra of the rabbit have epiphyses, absent in the dogfish and frog.

The transverse processes of the rabbit typically bear ribs. Short ribs occur in the dog-fish, but their homology with those of the rabbit is doubtful. The frog has no ribs.

The interneural plates are peculiar to the dog-fish in this comparison.

3. Compare the skull of the dog with that of the frog.

The Brain Case--

Of the frog is a cylindrical box, from which the otic capsules project conspicuously on either side. It contains only two ossifications in its cartilaginous substance (the sphen-ethmoid and the ex-occipital), being protected by the membrane bones, the parieto-frontals above and the parasphenoid below.

In the mammal it is enormously inflated, and the otic capsules are imbedded in its wall. There are supra- and basi- as well as ex-occipital bones; the para-sphenoid is (? entirely) gone, and its place is taken by the basi- and pre-sphenoids, and the lateral walls contain fresh paired ossifications, the ali- and orbito-sphenoids-- all cartilage bones. The sphenethmoid is perhaps represented in part by the ethmoid.

As a result of the inflation of the brain-case, the squamosal, which slopes downward and outward in the frog, and overlies the cartilaginous suspensorium (quadrate cartilage), has become a constituent of the brain-case wall, and slopes downwardly and in.

Jaw Suspension--

The point of attachment of the jaw has shifted outward, and the original suspensorial cartilage (the quadrate) has taken on a new and minor function as the incus of the middle ear-- the squamosal superseding it as the suspensory part.

Lower Jaw--

Distinct bones in the frog; one mass in the dog.

Otic Capsule--

Position as specified. One centre of ossification in the frog forming pro-otic; several fuse together and form periotic of the dog.

There is no bulla and no external ear in the frog.

Palate--

In the frog the posterior nares open into the front of the mouth. In the dog the maxillae and palatines send plates down and in (the palatine plates) to cut off a nasal passage from the rest of the buccal chamber, and carry the posterior nares back to the pharynx, thus cutting the vomers off from the mouth roof.

The pterygoids in the dog are much reduced, and do not reach back to the suspensorium.

The frog has no lachrymal bone.

-Syllabus Of Practical Work_

We would impress upon the student at the outset the importance of some preliminary reading before dissection is undertaken. No one would dream of attempting to explore a deserted city without some previous study of maps and guide-books, but we find again and again students undertaking to explore the complicated anatomy of a vertebrated animal without the slightest, or only the slightest, preparatory reading. This is entirely a mistake. A student should be familiar with the nomenclature of the structures he contemplates examining, he should have some idea of their mutual relations and functions, or his attention will inevitably be diverted by the difficulty of new names and physiological questionings to the neglect of his dissection, and that careful observation of form and mutual position which is the essential object of dissection. On the other hand, it is equally necessary-- perhaps more so-- to warn students against the bookish fallacy, and to assure them of the absolute impossibility of realizing biological facts from reading alone. Practical work can alone confirm and complete the knowledge to which the text-book is the guide. In scientific teaching it may sometimes be convenient for the thought to precede the thing, but until the thing has been dealt with the knowledge gained is an unsatisfactory and unstable possession.

For such dissection as the subject-matter of this book requires, the following appliances will be needed:--

(a) Two or three scalpels of various sizes.

(b) Scissors, which must taper gradually, have straight blades, and be pointed at the ends, and which must bite right up to the tips (or they are useless). Two pairs, small and large, are advisable.

(c) Forceps, which must hold firmly, and meet truly at the points.

(d) Two needles set in wooden handles.

(e) An ordinary watchmaker's eye-glass is very helpful, but not indispensable.

(f) A dissecting dish-- an ordinary pie dish will do-- into which melted paraffin wax has been poured, to the depth of, say, three-quarters of an inch, and allowed to solidify. (This wax may be blackened by mixture with lampblack. If the wax floats up at any time, it can, of course, be remelted. Or it may be loaded with lead.)

(g) A rough table or board (for the rabbit and dog-fish).

(h) Blanket pins, and ordinary pins.

(i) A pickle or other wide-mouthed jar, and some common, methylated spirit.

(j) A microscope, with low power of 1 inch or 1/2 inch, and high power 1/6 inch or 1/4 inch. Glass slips and cover glasses, and a bottle of very weak (1 per cent.) solution of salt.

Animals for dissection may be obtained from the recognised dealers, who usually advertise in such scientific periodicals as Nature, Natural Science, and Knowledge. Sinel (naturalist, Jersey) is the most satisfactory dealer in dog-fish in our experience; Bolton (Malvern) will supply Amphioxus through the post; frogs and rabbits may be obtained anywhere. The tame variety of rabbit is quite satisfactory for the purpose of dissection.

The following notes may possibly be of some use to the student; they follow the lines of work arranged by the author for the evening classes of the University Tutorial College, classes considerably restricted as regards time, when compared with ordinary laboratory workers. Most of the sections below occupied about three hours, but for a student working alone they are more likely to take four or five, and even then it is not probable that they will be so satisfactory as if performed under skilled supervision. There are many points extremely difficult to convey verbally which are elucidated at once by actual demonstration upon a specimen. Each of these dissections should be repeated, and it is well if a different condition of the type is selected for the repetition-- an old one if the first specimen was immature, a female if the first was a male.

-The Rabbit_

May be killed by chloroform, or potassium cyanide, or drowned. It may also be readily suffocated with house-hold gas. It should be killed immediately before use, as otherwise the gastric juice attacks the wall of the stomach, and the dissection is, in consequence, rendered extremely disagreeable. A very young rabbit is unsatisfactory as regards the genitalia, but otherwise there is no objection to a little one, and it has this advantage-- that it may be immersed more conveniently under water, in a large pie dish, for purposes of fine dissection. The external features of the animal should be examined: eyelids, whiskers and teeth, toes, anus, perineal space on either side of the same, urogenital opening, and position of the ribs, vertebral column, and limb girdles beneath the skin should be made out. Then the animal should be pinned out through the legs, the ventral surface uppermost, the skin opened up along the middle line from pelvic girdle to symphyses of jaw; separated from the body wall below by means of the handle of a scalpel, and turned back; and then the abdominal wall should be cut into and two flaps pinned back to expose its contents. Note the xiphisternum. The caecum and colon will be recognised (Section 16); the stomach, the right and left central, and left lateral lobes of the liver will probably be apparent; and the urinary bladder (especially if distended) in the middle line behind. Without any further dissection, but simply by turning the parts over, all the structures of the abdomen in Figure 1, Sheet 1, will be identified. Seek especially for and note particularly, the gall bladder, bile duct, and portal vein, pancreatic duct, sacculus rotundus, vermiform appendix, ureters (by pulling urinary bladder forward), genital ducts (looping over ureters), spleen, kidneys, and adrenals. The vena cava inferior is seen dorsally. The genital duct guides the student to the genital gland; if the subject is a male, the testes may be exposed by dissection, or by pulling the vas deferens gently the scrotal sac will be turned inside out, and the testes brought into view. The ovary lies exposed without dissection posterior to the kidney. Examine all this carefully, and make small sketches of points of interest-- the duodenal loop and the pyloric end of the stomach, for instance; the meeting of colon, caecum, and sacculus rotundus again; or the urinary bladder and adjacent parts. Note the dorsal aorta and vena cava and their connexions behind. (Compare figure of circulation.) Cut through pelvic girdle, and remove one hind leg, to see bladder and genital ducts better (compare Sheet 10). Wash away any blood that may flow. Turn all the intestines over to the animal's right, and see the dorsal aorta and vena cava inferior of the abdomen, the inferior mesenteric artery, and the spermatic (or ovarian) artery (compare, of course, with figure in book). In front, immediately dorsal to the spleen, is a variable quantity of lymphoidal tissue, which must be very carefully cleared to see the superior mesenteric and coeliac arteries. Separate Spigelian lobe from stomach, and look for vagus nerve descending by oesophagus, solar plexus around the superior mesenteric artery, and thrown up very distinctly by the purple vena cava inferior beneath, and the splanchnic nerve. To see the abdominal sympathetic behind, gently remove the peritoneum that lies on either side of the aorta; blood-vessels will be seen running in between the vertebral bodies, and the sympathetic chain, with its ganglia, made out very distinctly, as it runs across them longitudinally. Now cut oesophagus just in front of stomach, and cut the rectum, cut through the mesentery supporting the intestine, and remove and unravel alimentary canal; cut open, wash out, and examine caecum and stomach. Bleeding to a considerable extent is inevitable, chiefly from the portal vein. The liver had better remain if the same rabbit is to serve for the second dissection.

Second Dissection.-- Skin front of thorax and neck. Note subclavian veins running out to fore limbs-- avoid cutting these. Cut through ribs and remove front of thorax, to expose its contents; cut up middle line of neck, and clear off small muscle bands, to expose bloodvessels; pick away carefully whatever is left of thymus gland; make out structure of heart and blood-vessels, as described, in Chapter 3; note larynx and trachea. Now proceed to the examination of the nerves of this region. See phrenic nerve, by vena cava inferior, and between heart and lungs, and sympathetic, running over the heads of the ribs. By the common carotids will be found the large white vagus nerve, the greyish sympathetic, and a small branch of X., the depressor. Make out branches of X. named in text. The big white cervical spinal nerves will be evident dorsally. Clear forward into the angle between the jaw and the bulla tympani, to see XII. and XI.; IX. will be found, lying deeper, dorsal to the carotid artery and body of the hyoid. Compare with figure given of this. Skin the cheek, and see VII. running over it. Cut through malar and remove it; cut through lower jaw-bone and turn it back, to see the third branch of the fifth nerve on its inner side; examine the muscles of eyeball, and remove it, to expose the first and second branches of V.-- the latter is especially deep within orbit. Remove, open, wash out, and examine the heart. Shave off the dorsal wall of cranium, to expose hemispheres of brain, and then put the head in strong spirit for a week or so. With a second rabbit, this dissection may advantageously be varied by removing the lower jaw, cutting -up- [through] soft palate, and observing openings of the Eustachian tubes. [The tonsils (on the ventral side of the soft palate) must not confused with these.] The heart should also be cut out, washed out and examined (Compare Sections 38, 44.)

Third Dissection.-- (Before this is performed the mammalian skull should have been studied and examined.) Take the head of a rabbit, the brain of which has been hardened by spirit, and carefully remove cranium; be particularly careful in picking away the periotic bone, on account of the flocculi of cerebellum. It is difficult to avoid injury to the pituitary body embedded in the basisphenoid bone. Examine with the help of Sheet 8. Make the sections there indicated.

-The Frog_

May be killed by drowning in dilute methylated spirit, or by chloroform. Take a recently-killed frog, and examine a drop of its blood, spread out on a glass slip, under the microscope; compare it with your own. Before using the high power, put a cover glass over the object, of course. Scrape the roof of the mouth of the frog gently, to obtain ciliated epithelium; and mount in very weak salt solution-- the cilia will still be active. Squamous epithelium may be seen by the student similarly scraping the interior of his own cheek. Take a piece of muscle from one of the frog's limbs, tease out with needles upon a glass slip, and examine. To see the striations clearly, the high power will be needed. Compare a piece of muscle from the wall of the alimentary canal. Similarly examine nerve and connective tissue.

First Dissection.-- Pin out the frog in a dissecting dish, ventral surface uppermost, and cover with water. Open up the skin along the mid-ventral line. Note the large sub-cutaneous lymph spaces, the pelvic and pectoral girdles, and the anterior abdominal vein. Cut into the body cavity on one side of this latter, cut across in front of where the vein dips down to liver, and peel the body wall away from it. The xiphisternum will probably be cut in this operation. In early spring the females are greatly distended with ova, and the greater portion of the ovary may, with advantage, be removed. The oviduct is dead white then, and larger and much more in evidence than the (pinkish) intestine even. Turn over the viscera, and compare with Sheet 11; one lung is often found greatly inflated, and then projects back into the body cavity; the stomach is, in some cases, pushed forward and hidden behind the shoulder girdle. Observe the allantoic bladder, the spleen, gall bladder, portal vein, and pancreas. By squeezing the gall bladder gently, the bile duct will be injected with bile, and will be apparent if the stomach is turned over. The oesophagus, just in front of the stomach, should be cut through, and the rectum, and the mesentery and alimentary canal supported by it, removed. This will expose the urogenital organs. (Vide Figures given.) These vary greatly, especially in the females, at different seasons. The condition figured would be seen in late autumn, or winter. In spring females are often found copulating with males, and then the ovary itself is inconspicuous, while the lower part of the oviduct is enormously distended with ova, so as to be mistaken sometimes for the ovary by those who fail to note that the ova are enclosed by a thin semi-transparent skin (wall of oviduct). The vena cava inferior is seen between the kidneys and the renal portal vein beside the ureter. Cutting through the mesentery supporting the kidney laterally, the dorsal aorta is exposed, and on either side of it the sympathetic chain and rami communicantes, often tinged with black pigment. This black pigment is a frequent but variable feature of the frog's anatomy, and usually dapples or blackens the testes, and also sometimes darkens the otherwise pale pink arteries. Behind the kidneys the sciatic plexus also becomes visible. Careful drawings should be made. Cut off the head of the frog, shave off top of brain case, and put the head in strong spirit.

Second Dissection.-- A fresh frog is required. Pin out under water as before, and open up body cavity. Now carefully remove the muscle from the ventral portion of the shoulder girdle, to expose the clavicles and coracoids. Cut away xiphisternum, and then cut through clavicles and coracoids on either side, and remove ventral part of shoulder girdle, to expose the heart. Open out the cut portions of body wall and pin. The veins going towards the heart should now, with a little examination, be evident. Make out the external jugular, the innominate, and its two branches, and the pulmo-cutaneous and vena cava superior. Clear by carefully picking away any shreds of semi-transparent tissue. Make out, by feeling, the position of the hyoid body, and of its anterior cornua. Note the hypoglossal nerve (first spinal) running ventral to this, and the ninth cranial nerve, running parallel to it but dorsal to the hyoid-- hidden therefore by the hyoid, and reappearing in front. The vagus may also be made out less distinctly, running "postero-ventrally" towards the heart. By clearing the muscle by the rumus of the jaw, VII. may be seen, and the third branch of V., running across the jaw at about the middle of its length. Pick off the thin transparent pericardium from the heart very carefully, and proceed to cut away all the veins made out. The truncus arteriosus may then be followed up as it branches. Note all the branches shown in the figures in this book. The precise position of the vessels will vary to a certain extent with the attitude in which the frog is pinned. The cutaneous artery will prevent the student following up the aortic arch until it is cut; then the arch may be followed round until it meets its fellow to form the dorsal aorta. Note the sympathetic again. Make careful drawings of all this. Cut off lower jaw, and note posterior nares and Eustachian openings. If time allows, remove the heart, and examine by cutting open and washing. (Compare, Section 44) Remove eyeball, to see the first and second branches of the fifth nerve, and the Vidian (i.e. palatial) branch of the seventh.

Third Dissection.-- Read the account of the frog's skull carefully. Take the head of a recently killed frog and drop into boiling water for a minute. Then pick off, very carefully, muscle, connective tissue, nerves, and etc., to clear the cranio-facial apparatus; examine the bones, compare with figures given in this book, and draw. Take the head, which has been in spirit a fortnight or so, pick away cranium, and compare brain with figures given. Examine ventricles, by taking sections, after drawings have been made.

-The Dog-Fish-

First Dissection.-- Examine external characters, nasal grooves-- no internal nares-- fins, spiracle, scales passing over lips, and cloaca. Cut off tail below the cloacal opening. The males are distinguished by the large claspers along the inner edge of the pelvic fin. Open up body cavity. Usually this is in a terrible mess in the fish supplied by dealers, through the post-mortem digestion of the stomach. Wash out all this under a stream of water from a tap or water-bottle. Frequently the testes are washed out of the male in this operation and ova from the loose ovaries in the female. Now compare with figure given in this book, allowing for the collapse of the stomach, if it has occurred. Cut through the oesophagus and rectum, and remove alimentary canal from body; cut open and wash out the intestine, and examine spiral valve. Now make a careful examination of the cloaca and its apertures, and dissect away the peritoneum hiding the kidney. In the female find the opening of the oviducts in front of the liver. Remove liver, and cut off body now behind pectoral fin. Before throwing tail and hinder part of body away, note the myotomes of body wall, the notochord and vertebral body, neural canal, and, in the tail, the haemal canal. [(See {Section 9 the Dog-fish})]

{Lines from First Edition only.} -The relation of the vertebral bodies to the notochord may be very well seen by taking successive slices, about one-tenth of an inch thick, through the vertebral body. The cartilage is hard and semi-transparent, the notochord jelly-like, least at the centres of the centra, and at a maximum intervertebrally.-

Second Dissection.-- Place the head with the ventral side downward, skin all the dorsal surface as yet unskinned. Refer to book for precise position of the anterior cardinal sinus, and then cut down through body wall into this just over gill slits. The tenth nerve will become visible, with its "slit" branches athwart the floor of the sinus. Clear to make this more evident, and make out its lateral line and visceral branches, and the ninth nerve.

{Lines from Second Edition only.} [The pharyngo-branchials may be felt beneath the sinus. Run a seeker from the dorsal aorta to the efferent branchials.]

Proceed now to orbit, and, without any dissection beyond the removal of skin, make out recti and oblique muscles of eyeball, and the optic, third and fourth nerves. Cut through these structures carefully and remove, exposing nerves seven, and five, as described and figured in the text. Examine the otic capsule by taking successive slices through it to show the labyrinth of the ear. -Remove the dorsal wall of the skull to obtain a dorsal view of brain. If this is sufficiently hard, examine it; if not, return it to spirit for a more convenient occasion.- [Examine brain.]

-Amphioxus_

Two specimens of this type should be obtained. It should be examined entire by the naked eye and with the low power of the microscope. Immersion, in glycerine will render it more transparent; or it may be cleared with oil of cloves, put up temporarily in that, or permanently in Canada balsam. One specimen should then be pinned out in the dissecting dish, ventral side uppermost, and the atrium opened to expose liver and pharynx. A part of the pharynx may be examined with the low power to see the form of the gill slits. The second specimen should be soaked in turpentine for some time, and then dropped into melted paraffin wax. Transverse sections may then be cut with a razor, the paraffin wax removed from these by solution in turpentine, the turpentine in its turn dissolved out by alcohol, and the sections, after immersion in oil of cloves, may be transferred to Canada balsam for examination and preservation. This work should not be attempted until some practical histological work has been done in botany, and it may be altogether avoided by the purchase of stained and mounted sections.

-Development_

Laboratory work in this portion of the science is not usually undertaken by elementary students of biology, but the reader will probably find it helpful, in the realization of the facts given in this book, to look out for frog spawn, in February and March, and to catch and examine tadpoles of various sizes. A small dissecting dish may be made by pouring melted paraffin wax into one of those shallow china pots chemists use for cold-cream, and tadpoles may be pinned out with entemologists' pins and dissected with needles. But this is a work of supererogation. Partially incubated hen's eggs may be obtained at a small cost almost anywhere, and the later stages profitably examined and dissected under warm water. For a clear understanding of the allantois and amnion, this last is almost indispensable. A few microscopic slides of sections of embryonic chicks should also be compared with our rough diagrams.

-{Key for Dissection Sheets, and Abbreviations.}_

Sheet 1

Figure 1. Main facts of the Rabbit's Anatomy (diagrammatic). an., anus. a.ao., arch of the aorta. au., auricle. a.r., ad-renal body. br., brain. b.d., bile duct. brch., bronchus. cd.st., cardiac end of stomach. co., colon. cae., caecum. ddnm., duodenum. d.ao., dorsal aorta. dia., diaphragm. ep., epiglottis. g.d., genital duct (either sex). il., ileum. in.art., innominate artery. k., kidney. lg., lung. lv., liver. l., larynx. l.s.c., [l.c.c.] left common carotid artery. m., mouth. na., nasal passage. oes., oesophagus. p.v., pyloric valve. p.d., pancreatic duct. pt., peritoneal cavity. r., rectum. st., stomach. [stm., sternum.] s.r., sacculus rotundus. s.c., spinal cord. tr., trachea. ur., ureter. ur.b., urinary bladder. v.b., a vertebral body. v.ap., vermiform appendix. v.v., [v.p.] velum palatium. v., ventricle of heart. v.c.i., vena cava inferior.

Figure 2. The Liver (diagrammatic). g.b., the gall bladder. r.l., r.c., l.l.., l.c., right lateral and central, and left lateral and central, lobes respectively. sp., the Spigelian lobe (fits into angle of stomach and oesophagus).

{Illustration: Diagram Sheet 1.}

Sheet 2

Figure 1. The Rabbit's Circulation (see footnote to Section 45).

(Throughout l. indicates left, r. right. Vessels without r. or l. prefixed are median.)

-[* The figure is inaccurate at one point; l.c.c. should spring from the base of inn. See Sheet 9.]- {First Edition only text}

ao.a., aortic arch. au., auricle. az.v., (p.c. in Figure 2), azygos vein. c.c., common carotid. c.il.a., common iliac artery. coe.a., coeliac artery. d.ao., dorsal aorta. e.il.v., external iliac vein. e.ju., external jugular vein. f., femoral artery. h.v., hepatic vein. inn., innominate artery. in.j., internal jugular vein. i.il.a., internal iliac artery. i.il.v., internal iliac vein. k., kidney. lv., liver. l.g.v., lienogastric vein (portal). m.v., mesenteric (portal system). p.m.a., posterior mesenteric artery. p.v., main portal vein. p.a. pulmonary artery. r., rectum. r.a., renal artery. r.v., renal vein. s.v., and a., spermatic (or ovarian) vein and artery (to genital organ). s.mes.a., superior mesenteric artery. s.-cl.a., subclavian artery. s.-cl.v., subclavian vein. v.c.s., vena cava superior. v.c.i., vena cava inferior. v. or vn., ventricle.

Figure 2. Figure of Circulation (simplified) illustrating certain points in development to be referred to later.

Figure 3. Respiration. See text, Section 41.

Figure 4. Blood. See text, Section 35.

{Illustration: Diagram Sheet 2.}

Sheet 3

Histological Figures, 1.

{No numbers I., or II.}

Figure III. An amoeba.-- n., nucleus. ns., necleolus. c.v., contractile vacuole.

Figure IV. Embryonic tissue from the blastoderm of a chick.

Figure V. Columnar epithelium.-- g.c.1, g.c.2, g.c.3, successive phases in the development of a goblet cell.

Figure VI. g.end., is geminating endothelium; the cells divide and apparently drop off to become white corpuscles in the lymph current. sq.end., squamous endothelium from the mesentery. sq.ep., squamous epithelium (from the mucous membrane within the cheek). st., are opening (stomata) communicating between the lymphatics in the mesentery and the peritoneal (coelomic) space.

Figure VII. Ciliated epithelium from the roof of the frog's mouth.

Figure VIII. Forms of glands.-- g.ep., is a gastric gland from the stomach; trs., below, is cross section. This is one of the simplest types of gland. s.g., a sweat gland, is also a simple tube, but convoluted below. r.g., is a racemose gland, such as the pancreas, Brunner's or the salivary glands.

The kidney, we shall see later, is simply an aggregate of branching tubuli (Sheet 7).

Figure IX. A duodenal villus.-- lac., the lacteal. v., the vein.

Figure X.a. Diagram of liver structure.-- b.d., the inter-lobular bile duct. h.a., the hepatic artery, bringing blood to oxygenate and nourish the liver tissue, and similarly distributed. h.v., the hepatic vein taking blood from the liver to the heart, its twigs commencing in the lobuli (intra-lobular). lb. lb., lobuli. p.v., the portal vein bringing blood, from which substances are to be elaborated, into the liver, and breaking up between the lobuli (inter-lobular).

Figure X.b. A diagram of the appearance of an injected liver lobule as seen in section under the microscope.

{Illustration: Diagram Sheet 3.}

Sheet 4

Histological Diagrams, 2.

Figure XI. A blood capillary. White corpuscles are migrating through the walls into the tissues (compare Section 66).

Figure XII. Hyaline cartilage (Section 64).

Figure XIII. c.c., connective tissue corpuscle. w.i.f., white inelastic fibres. y.e.f., yellow elastic fibres.

Figure XIV. Botryoidal tissue (Section 66).

Figure XV. Development of a fat drop.-- f.d., fat drop, in a connective tissue corpuscle; c.c., in the formation of adipose tissue (Section 67).

Figure XVI. Diagrammic cross section of a long bone.-- b.c., bone corpuscle in a lacuna. H.v., Haversian vessel (in the Haversian canal) surrounded by concentric lamellae of bone, c.l., and together with these and zones of bone corpuscles, called a Haversian system. i.l., inner lamellae. m.c., medullary canal full of yellow marrow. o.l., outer lamellae. p.o., periosteum.

Figure XVII. To illustrate bone development (Section 71).

Figure XVIII. Dentition of rabbit, incisors 2/1, canine 0/0, premolar 3/2, molar 3/3.

{Illustration: Diagram Sheet 4.}

Sheet 5.

Diagram of the Rabbit's Bones. To be compared with the real things.

D and D' show the fore and hind limbs, to illustrate their homology. D is in the embryonic position. The radius and tibia are, at an early stage in development, on the anterior edge of their respective limbs; the ulna and fibula, posterior; the former are spoken of as preaxial in position, the latter as postaxial. But in the adult the humerus is twisted so that the proximal end of the radius lies at the outer side of the elbow, whence it crosses the ulna, so that its distal end is inside, while the femur is also twisted round, so that the entire tibia is internal.

Figures 1 and 2. -Limbs.-- a.c., acetabulum. acr., acromion. as., astragulus. c., carpus. ca., calcaneum. co., coracoid. [coty., cotyloid bone.] fb., fibula. fe., femur. g., glenoid cavity (for head of humerus). hd., head of femur. hum., humerus. i., ilium. is., ischium. m.c., meta-carpals. na., navicular. o., olecranon process of ulna. o.f., olfactory fossa. pb., pubis. r., radius. u., ulna.

Figure 3. -Sternum.-- Mb., manubrium. r1., r2., and etc., sternal ribs. st., sternebrae. xi., Xiphisternum.

Figure 4. Vertebrae.-- At., Atlas. Ax., axis. c., [b.] centrum. C.V., caudal vertebra. c.v., [Cer.V.] cervical vertebra. ep., epiphysis. f.r., fused rib (in cervical vertebrae). L.V., Lumbar vertebra. m., metapophysis (of lumbar vertebra). n.a., neural arch. n.s., neural spine. r., rib. S.V., sacral vertebra. [T.V., Thoracic.] tr.p., transverse process. v.a.c., vertebrarterial canal. z., zygapophysis.

{Illustration: Diagram Sheet 5.}

Sheet 6.

The Skull of Canis.*-- 1. Dorsal. 2. Ventral. 3. Right Lateral Aspect. 4. Section a little to the left of the nasal septum. 5. Lower jaw (smaller) 6. Hyoid apparatus.

{Lines from First Edition only.} -*A Fox in this case. The skull is quite like that of a Dog, but it has the advantage of more distinct sutures between the bones.-

a.n., anterior nares. a.s., ali-sphenoid. b.h., body of the hyoid. b.o., basi-occipital. b.sp., basi-sphenoid. c., condyle of the skull. {c.1, c.4, canines.} c.f., condylar foramen (for XII.). c.h., cerato-hyal. E.f., Eustachian foramen. e.h., epihal. -e.n., or a.n., the anterior nares.- e.o., exoccipital. eth., ethmoid. e.t., ethmo-turbinal. f., frontal. f.l.a., foramen lacerum anterius. f.l.m., foramen lacerum medium. f.l.p., foramen lacerum posterius (for IX., X., XI.). F.M., or f.m., foramen magnum. f.o., foramen ovale. f.r., foramen rotundum. {i., incisors.} ju., jugal. m., molars. m.t., maxillo-turbinal. mx., maxilla. na., nasal. n.t., nasal turbinal. o.f., optic foramen. o.s., orbito-sphenoid. p., or pal., palatine. pa., parietal. p.m., pre-maxilla. p.m.1, p.m.4, premolars. p.n., posterior nares. p.sp., pre-sphenoid. pt., pterygoid. s.h., stylo-hyal. s.m.f., stylo-mastoid foramen (for VII.). s.o., supra-occipital. sq., squamosal. s.t., sectorial tooth. t.h. thyro-hyal. vo., -black line indicating position of- vomer. z.p., zygomatic process of squamosal.

{Illustration: Diagram Sheet 6.}

Sheet 7

Figure 1. Striated muscle fibre (of the Rabbit), ruptured to show sarcolemma. e.p., its end plate. K.m., membrane of Krause. n., nucleus. nv., nerve. sc., sarcolemma. s.e., sarcous elements.

Figure 2. Cardiac muscle.

Figure 3. Unstriated muscle fibres.

Figure 4. Diagram of the Skin. b.v., blood vessel. d., areolar tissue of the dermis (mesoblastic). s.c., stratum corneum, and s.m., stratum mucosum of the epidermis. s.g., sweat gland. t.c., tactile corpuscle.

Figure 5. To illustrate Kidney structure.-- a.b.v., and e.b.v., afferent and efferent blood-vessels, of which the latter go to break up upon the tubli. B.c., one of Bowman's capsules of the cortex; ur.t., the uriniferous tubule running from it into the medulla, where it loops and branches; around it branches a blood-vessel, of which the latter go to break up upon the tubuli. c., cortex. g., glomerulus, a knot of blood-vessels in the capsule. m., medulla. p., pelvis. ur., ureter.

The water of the urine is probably filtered off in the capsule, the urea and other salts secreted by the tubuli.

{No Figure 6.}

Figure 7. The Auditory structures of the Rabbit (diagram). See text,