Part 22

The breast-bone or sternum (fig. 8, p. 13) of birds shows the same relation to the power of flight that is shown by so many, if not by all, parts of the skeleton. It is relatively a very large bone, and is in all perfectly flying birds furnished in the middle line, below, with a strongly marked keel, the presence of which has given its name to the great group of birds called carinates. The ostrich tribe, from whose sterna the keel is absent, are termed ‘ratite,’ or ‘raftlike.’ The reason for the keel is the attachment of the great pectoral muscle, which is the most important muscle of flight. The sternum often offers useful characters to the systematist. The surface of the bone is sometimes in various degrees fenestrate, or more or less deeply incised, the one condition being an exaggeration of the other, and both the conditions being due to defective ossification. The sternum is attached to the vertebral column by the ribs, which are well developed in all birds, but vary very much in number. A highly characteristic feature of the ribs of birds is a small bony projection of the hinder margin of a certain number of them, called the uncinate processes. These are present in all birds, with the single and remarkable exception of the South American Screamers (Chauna, Palamedea), a group of birds occupying a rather isolated position, and showing resemblances to a great many different groups.

Pelvis.

c, d, ilium; 63, ischium; 64, pubis; 65, femur; 66, tibia; 67, fibula; 68, tarso-metatarsus; i.-iv. digits with phalanges numbered.]

The hind limbs are attached to the vertebral column by means of a considerable bony structure known as the pelvic girdle (fig. 13). This mass of bone is in reality composed of three pairs of elements, though they are in the adult strongly compacted together. The main bone, which is firmly attached to the vertebral column, is the ilium; with this is almost completely fused the ischium; the very slender pubis is to a large extent free from these bones. The pelvis is in its form one of the most characteristic of the bones of the bird’s skeleton. In other animals the three bones are present, but they are directed away from each other; in the bird, as already described, the pubis is directed backwards, parallel to the ischium; in correspondence, perhaps, with its position it has become a feeble bone, and has but few muscles attached to it. The interest of the matter, however, is mainly in the fact that among the extinct Dinosaurs, a race of mesozoic reptiles, there were some in which the pelvis had a very bird-like structure, with the same feeble and recurrent pubis. This has been urged as a mark of affinity between the Dinosaurs and birds. The several bones of the pelvis are free from each other at the extremity, or almost so, in all the Ratites, and in the Tinamous, which are supposed to bear some relationship to the Ratites. The fact is interesting as being an example of the retention of a character by one group of birds which is only transitional and embryonic in another, for in all young birds the bones of the pelvis are separate; it is not until some time before hatching that they become fused together as we see them in the adult.

Hind Limb.

At first sight there appears to be a considerable difference between the fore limb and the hind limb. In both there is a long proximal bone, called humerus in the one case and femur in the other, followed by a pair of bones--the tibia and fibula--corresponding to the radius and ulna of the fore limb. But in the hind limb (fig. 13), the foot proper, consisting of metatarsals and phalanges, appears to come immediately after the tibia and fibula. In a sufficiently young bird, what is the apparent lower end of the tibia, and what is equally apparently the upper end of the metatarsus, are detachable; these two halves which are thus detachable are the tarsus, which is the equivalent of the carpus of the wing. The lower bone of the leg is on this account usually spoken of as the tarso-metatarsus. The lower part of this bone is made up of three fused elements, the separation of which from each other is clearly apparent at the lower end of the bone, where the phalanges are attached. In the Penguins the three bones are separated by grooves of a very marked character throughout. In some birds there is a fourth toe, the hallux; in these cases there is a small separate metatarsal loosely fixed to the lower end of the large conjoint metatarsals.

Gizzard and Alimentary Canal.

o, orifice of duodenum; a, end of proventriculus; cd, muscular part of gizzard.]

The gizzard (fig. 14) of the fowl is simply a part of the stomach which has especially hard and muscular walls, the other half remaining soft in texture; this latter is termed the proventriculus, and into it open the mouths of glands which secrete the digestive juice of the stomach. But the muscular part of the stomach--the gizzard--has to grind down the frequently hard food of the bird, so it has not merely a strong wall made of muscle, but also a very tough lining; the whole organ, therefore, forms a highly efficient mechanism for crushing and grinding the seeds and other hard vegetable food which is swallowed. It is rendered more useful still for this purpose by the pebbles which every bird takes care to swallow. The true and singular stories about the varied contents of an Ostrich’s stomach are founded upon the fact that, like other birds, it picks up stones, and with them occasionally other objects. But all birds do not possess a hard gizzard; in Hawks and fish-eating birds the walls are thinner, and the organ is flaccid instead of being rigid. By a very curious and unique exception certain Tanagers, a race of large, often bright-coloured, American, finch-like birds, have nothing at all that can be compared to the gizzard of other birds; this part of the alimentary canal is totally wanting. Now the difference between the gizzard of the grain-eating fowl and the flesh-eating hawk is chiefly a matter of diet. The celebrated anatomist, John Hunter, who lived in the last century, and wrote so much about the anatomy of all kinds of animals, including birds, found that he could feed a soft-stomached bird into one with a hard gizzard, and vice versâ.

We can pass briefly over the rest of the alimentary system, which does not vary a great deal in different birds. The intestines are always rather short, and are diversely coiled, the method of coiling being often characteristic of a particular group. A good way down the intestine are a pair of cæca, which may be entirely absent, as in the Hornbills, for example; and if present may be extremely short, as in the Sparrow, or very long, as in the Ostrich; various intermediate degrees exist. As in all vertebrated animals, two glands pour their secretion into the intestine; these are the pancreas and the liver. The secretion of the liver is the bile; this fluid is accumulated as it is formed in a largish bag--the gall-bladder, in those birds which possess one. Shakespeare used the epithet ‘pigeon-livered,’ which meant literally the absence of a gall-bladder; but, oddly enough, there are some kinds of pigeons which have a gall-bladder, while others, like the common pigeon, have not. The intestine ends in the cloaca, which is the common chamber into which the urinary and generative organs also open.

Tongue and Teeth.

In the inside of a bird’s mouth we find only one of the two things that we might expect to find: there is a tongue, but no teeth. We shall come back to the teeth immediately. The tongue is not so useful among the majority of birds as it is in most mammals. But some do make use of it to a great extent. If you watch a parrot eating its food, you will observe that its thick and fleshy tongue is of the greatest assistance in helping it to manipulate the pieces of food--to extract, for instance, the kernel from a seed or nut. It plays exactly the same part as it does with us. In one kind of parrot, called the ‘Brush-tongued Parakeet,’ the tongue is frayed out at the free end into a brush-like extremity. And there are some small birds, which peck at flowers and live upon honey, in which the tongue is thin and delicate, and frayed out in the same way; this allows them to suck up the juices of the flower. In the Hummingbird the tongue is rolled up so as to form two tubes running side by side, and the same power of sucking up juices is acquired by this means, which, curiously enough, is exactly paralleled by the proboscis of the butterfly. In other birds the tongue is sometimes merely a thin, flat, horny projection, and in others, again, it is just not absent altogether.

A little reflection about the habits of birds will show that they really do not want teeth; and we know that Nature is a most rigid economist: nothing superfluous is allowed in the body. Even rapacious birds like Owls and Hawks have no teeth, because they have a powerful beak and claws, with which the food may be as effectually torn to pieces. Birds such as the Pigeon, which feed upon grain, possess a gizzard--which we have had something to say about already--that performs effectually the function of a mill, grinding into a powder the hard grains of wheat and other seeds which the bird swallows. Nevertheless birds once did possess teeth. In earlier times of the history of this earth there were some birds whose jaws had as formidable a range of teeth as the mouth of many reptiles. They were fish-eaters, and have been named Hesperornis and Ichthyornis. The first was something like a Diver in shape, the latter more like a Gull. A still more ancient bird, the oldest form of bird known to us, the Archæopteryx, had also toothed jaws. In fact, in the old days it was the rule for birds to have teeth, whereas now it is the rule, without a single exception, for birds to be toothless. Perhaps these ancient and extinct forms had some corresponding disadvantage when compared with their modern representatives; their teeth and claws, for example, may have been less effective. But although there is no bird now living which has real teeth, traces of these organs have been discovered in the young embryos of certain birds, which seems to be an absolute proof that they, at any rate, had for their first parents toothed birds. But although modern birds have no teeth, with enamel, dentine, and so forth, all complete, the horny beak has occasionally ridges which to some extent play the part of teeth. The inside of the Duck’s mouth is rough with such ridges, which occur also in some other birds. The large Flamingo was for some time regarded as a long-legged and awkward Duck that had partially adopted the habits of a Stork, partly on account of the fact that the inner edges of the beak were ridged in a fashion exactly like that of the Duck. But it happens that there is a Stork, a true Stork, in India, whose scientific name is Anastomus, which has similar ridges. Ducks feed to some extent upon shellfish, which the roughened edges of the beak are well suited to crush. The replacement in the course of ages of true teeth by horny teeth is seen--a curiously parallel case--in the Duck-billed Platypus of Australia, which has when adult horny plates instead of teeth, but when young has real teeth.

Heart.

As with all vertebrated animals, birds have a centrally placed heart, with which are connected arteries and veins, the two systems of tubes being connected at the ends farthest away from the heart by minute vessels--the capillaries. In relation, no doubt, to the intelligence and activity of birds, as compared with their slower relatives, the reptiles, we find a heart of much more perfect organisation. There are four distinct chambers, as in the mammal, so that the arterial and venous blood are separate, and do not commingle. The two sides of the heart are only in indirect communication by way of the arteries and veins and capillaries. The left ventricle gives rise to the aorta, which is the great arterial trunk of the heart; this divides into the carotid and other arteries, which supply the entire body, with the exception of the lungs. The blood, which is sent out through this vessel by the contractions of the ventricle, permeates the system generally, and is then collected into a series of veins, which ultimately unite into two great veins, the venæ cavæ in front, and a large vein situated posteriorly, the inferior vena cava. These pour the blood back into the right auricle, whence it passes at once to the right ventricle. From the right ventricle it is driven into the lungs, whence it is returned to the left auricle, and so into the left ventricle to renew the circulation. The two chambers of each half of the heart are guarded from each other by valves, which only allow the blood to flow in the proper direction, as stated in the above brief description of the course of the circulation. It is a curious fact that the valve which separates the right auricle and ventricle is a completely muscular structure, while the other is membranous. Moreover, it does not form a complete circle, but is deficient upon one side of the orifice. The interest of this fact is not merely in its abnormality, its divergence from what one would expect, but in the resemblance which is thus shown to a group of mammals, the Monotremata. This group includes only the Duck-billed Platypus of Australia and the spiny Anteater (Echidna) of the same continent and New Guinea. In both of these animals the heart valve in question is also largely muscular, and does not entirely encircle the opening from the auricle. These two mammals also, as everyone knows by this time, have the strange habit for a mammal of laying eggs, which is one among some other reasons which once led naturalists to place them in the neighbourhood of birds. The egg-laying, of course, is not distinctive, since reptiles have the same way of bringing forth their young; and as to the heart valve, it is rather to be explained by the fact that both types of animals are low in the scale of their respective groups, and therefore both approach a common ancestral form.

Voice Organ.

g, tympaniform membrane.]

a, b, c, e, f, intrinsic muscles; d, sterno-tracheal muscle.]

i, pessulus; h, vibrating membrane; g, membrana tympaniformis.]

By their voice, too, birds are distinguished from the rest of the animal creation. Though there may be legends of singing serpents and of talking monkeys, a harsh scream or a growl is the only manifestation of the emotions through the voice which exists until we arrive at man. Among birds, the possession of a melodious voice is limited to that group which we term the Passeres. Other birds can scream or utter a dull note, while many are mute. So flexible is the voice organ of these creatures that they are the only animals that can imitate human speech. Here, however, it is not only the Passeres which can imitate the essential attribute of man. The Parrots, of course, are always supposed to be the birds which can talk, but this is far from being the truth. The hoarse utterances of most Parrots are left far behind in clearness of sound and correctness of imitation by the little Indian Mynah, which may be usually seen at the Zoological Gardens, and heard to speak. But the Parrot cannot sing. These are the only two groups of birds which have so elaborate and flexible an organ of voice. From this it might be inferred that some peculiarities of mechanism would distinguish the organ in question of these birds, and that is what we actually find to be the case. But, oddly enough, it is not only those birds which have a beautiful voice whose voice organs are so elaborate in structure. The harsh croak of the Raven issues from a syrinx which is as delicately fashioned as that which allows of the exquisitely varied tones of the Nightingale. The word ‘syrinx’ has been mentioned; that is the technical term for the voice organ of the bird, which is formed from a part of the windpipe, as in man and the mammalia, but from a different part of that tube. In man and in mammals the voice organ is placed in the throat just a little way down, at the prominence often spoken of as ‘Adam’s apple.’ This is a wider part of the tube, with larger rings of cartilage, which contains a pair of tightly stretched membranes that can be made to vibrate and cause a sound. In the bird, the voice organ is situated farther down, at the very point where the trachea forks into the two bronchi, one for each lung. Here are figures which illustrate the voice organ of a singing-bird (figs. 15, 16, 17). At this forking of the trachea the rings of the tube, which are of gristle or cartilage, become somewhat different in form. In the middle is a piece, which is often converted into bone, like the ‘three-way’ piece used to fix together the stick and the hoop of cane of a butterfly-net. To the upper side of this, and therefore within the tube, and directed upwards, is a little crescent-shaped piece of membrane (h, fig. 17); this can be set vibrating by the stream of air passing up and down the windpipe. At the sides of the syrinx there are shown in the figure (fig. 16) three pairs of muscles; these when they contract shorten the syrinx, and of course produce alterations in the note, just as the shortening of the tube in a cornet alters the sound. In many passerine birds, and in most other birds, there is only one pair of these muscles; but the Parrots agree with the passerines in having several pairs of muscles, and therefore a more easily alterable syrinx. In a good many birds there are no muscles at all in this place; for example, in the Storks, which have not by any means a flexible voice. The syrinx, in fact, is one of those organs which show a great deal of difference in different kinds of birds. But it is never entirely absent, though rather rudimentary in the Ostrich. The Australian Emu has a curious way of producing its sounds which is not found in any other bird. The cock and hen Emus can only be recognised by their voice, which is duller in the hen and sharper in the cock. When the bird is uttering its note, it seems almost to come from somewhere else, and not from the throat of the bird; the bird is something of a ventriloquist. The sound, which is a low bellow, is produced by a bag of skin opening into the windpipe some way up the neck; a current of air passing down the tube is believed to set the air in this bag in vibration, just as the air in a key may be caused to vibrate by blowing over its edge. Generally speaking, the windpipes of birds are straight tubes running to the lungs by the shortest route; but in the Cranes, and in a few other birds, the pipe is coiled upon itself once or twice, and the coils are even hidden in an excavation of the breast-bone. The increased length of tube gives a louder and more resonant note, such as we know characterises the Crane.

Lungs and Air-sacs.

It is not only by virtue of their powerful muscles and stiffened fore limbs that birds can fly. The body is rendered lighter in proportion to its bulk by air-cavities, which permeate everywhere, even into the substance of the bones. So thorough is this aëration in the Screamer of South America, that when the skin of the recently dead bird is roughly pressed it crackles. Curiously enough, there seems to be no very definite relation between the degree of thoroughness to which the aëration of the body is carried out and the capacity for flight. The Screamer, that has just been mentioned, is fuller of air-cavities than the Frigate-bird, in which the art of flying is carried to the highest extreme--the ‘triumph of the wing,’ as Michelet says in ‘L’Oiseau.’ Anyone who has the opportunity of dissecting a Hornbill will be struck by the large and abundant air-spaces between the muscles. This applies even to the Ground Hornbill of Abyssinia; and yet the latter, as its name denotes, lives upon the ground, while the flight of other hornbills is heavy and most unsuggestive of lightness of body. These air-spaces are in direct communication with the windpipe. It is much easier to understand their arrangement by the actual dissection of a bird. We must first get a notion of the position and form of the lungs, which differ very much from the lungs of other animals. In a rabbit, for example, or any other mammal, the lungs lie freely on each side of the heart, and are capable of being pushed here and there after the body is opened, and of much expansion and diminution of volume during the movements of respiration. But the lungs of all birds are tightly fixed to the wall of the chest cavity, being, as it were, moulded on to the ribs and vertebræ; when they are carefully picked away from their place, they retain the impressions of the bones which they touch. There is no great possibility here of independent movements on the part of the lungs. Respiration is effected in a totally different manner; it is, in fact, bound up with the mechanical filling of the air-spaces. Each of the two lungs is contained within a large compartment, which is bounded externally by an obliquely disposed septum, often spoken of, on account of its direction, as the ‘oblique septum.’ Others call it the diaphragm, imagining that it is the equivalent of the diaphragm in the mammal, that partly fleshy, partly tendinous plate which shuts off the cavity of the chest, in which lie the heart and lungs, from the cavity of the abdomen, in which lie the intestines, stomach, and liver. Now, this oblique septum does not by any means closely invest the lungs; on the contrary, a deep space is thereby shut off, at the bottom of which are the lungs. This cavity is subdivided by two partitions into three separate compartments. It requires a very skilful manipulation to show the fact, but it can, with care, be demonstrated that each of these compartments is lined by a delicate membrane, which is continuous with the lung, and is actually a kind of bubble, as it were, blown out of the lung; these delicate sacs are the air-sacs. There are altogether nine of them, but all these sacs do not lie within the cavity bounded by the oblique septa. The largest pair of all the abdominal air-sacs project into the body cavity far behind the gizzard. Now these sacs are fairly easy to see in a dissection; but it is not so easy to make out that they are all of them, except the middle two, connected with a system of ramified air-spaces which, as already said, permeates the body generally, lying among the viscera, between the muscles below the skin, and deep into the actual interior of the bones. But though it is difficult to see this by a dissection, it is easy enough to prove it by inflating them. If a syringe is passed down the windpipe and tied carefully into it, so that no air can escape at the sides, and air is blown down the tube, the passage of the air into the skin and other parts can be followed; if a bone be cut across, the air can be noticed to issue from the cut surface; and if the experiment be varied by using a coloured fluid instead of air--which is pumped in by a syringe--the fluid can be seen to ooze from the end of any bone or muscle that has been cut across. A bird, therefore, when it takes in a deep breath, not only supplies its lungs with fresh air, but fills its whole body with the superfluous air. It has been proved that a bird can continue to breathe if it be held under water, and only the end of a broken limb allowed above the surface; for, as all the spaces of air are in communication with the lungs, they (the lungs) can obviously be as conveniently filled from one end as from the other. When you are bathing, and take a very deep breath as you are swimming, you can detect a sensible increase in the buoyancy of the body; in a bird, of course, the difference is enormous, after the sacs are filled, from a condition of comparative emptiness. The way in which a bird breathes is different from the way in which a human being breathes. There is, of course, the essential resemblance that is shown between all animals that have definite organs which are set apart for respiration: the feathery gills of the marine worms, the closely set branchiæ of the fish, the lungs of the bird and of the mammal, are all constructed upon one plan, so far as essentials are concerned. In all of them blood-vessels are brought into close relation, though not into actual contact, with water or air containing oxygen. The blood-vessels are separated from the water or air by the thin membranes of the lungs or gills, through which the oxygen can pass in to the blood, and the carbonic acid and effete gases can pass out; it is this exchange which is the essential act of respiration. We cannot, however, in this book pretend to go into general matters of this kind, which would take us too far from the subject at hand; but anyone who would pursue this further can consult Professor Huxley’s ‘Elementary Physiology,’ or any other elementary text-book upon physiology. When a mammal--a human being, for example--breathes certain muscles are called into play. If a person is watched, it will be seen that the chest expands during inspiration, and that its calibre diminishes during expiration. What happens is this. The lungs are contained in a cavity which contains no air. This cavity can be increased in size in two directions. When the ribs are moved out--which they can be by the movements of the muscles called intercostal, which lie between them--the cavity of the chest from before backwards is evidently enlarged. On the other hand there is the diaphragm, which we have already spoken of as bounding the chest cavity below. Now this diaphragm is muscular, with a tendinous centre. When the muscles contract, like all muscles do, the surface of the diaphragm, which was before rather convex towards the chest cavity, becomes more flat; hence the cavity lying above it, i.e. the chest cavity, becomes larger in a downward direction also. When it is increased in this way by the action of the two separate sets of muscles, some space--more space than before--is left between its walls and the lungs which lie within it; it follows, therefore, that, as there is no air in the cavity, the pressure of air outside the body forces more air into the lungs, because there is no counterbalancing pressure to prevent this. The principle is the same in the bird, but the details are different. If you will turn again to the bird’s skeleton, you will see that the backbone and ribs and sternum form a bony box, which is jointed in the middle; this acts precisely like a pair of bellows: the bones at top and bottom represent the wood, and the soft intervening leather of the bellows is represented by the muscles which lie between, and which connect the sternum with the abdomen and with the ribs. When these muscles contract, the sternum is obviously brought nearer to the backbone, and air is expelled from the inside; when they are relaxed, a vacuum is created and air rushes in. The air-spaces, then, are really ramified tags of lung which have no blood-vessels in their walls, and are therefore not meant for respiration, but serve as reservoirs of air, lightening the body of the creature. It is curious that birds are not the only animals which possess expansions of lung that are apparently useless for breathing purposes. The lungs of the Chameleon have quite similar sacs appended to them. There is, it is true, no such complicated a ramification as that which we find in the bird, but still there is no doubt that the structure is of the same nature. It looks almost like a first step in the path towards a bird. Very possibly the extinct Pterodactyles, which flew through the woods of the middle ages of the earth, had bodies lightened in the same or a similar way; for we know that their bones have thin walls, the large cavity of which in all probability contained air-sacs. Even some of the jumping Dinosaurs, to which reference has already been made, seem to have possibly had lungs constructed on the bird type. We see, therefore, that even where a bird is, so to speak, most characteristically a bird--in the subsidiary mechanisms of flight--it betrays a likeness to the comparatively grovelling reptile, letting alone the aërial and more bird-like Pterodactyles.

Brain.

The brain of birds is large in proportion to the body, thus contrasting with that of the unintelligent reptile. From some tables on the matter which have been published, it appears that, if weight of brain goes for anything, the goldfinch is one of the most intelligent of birds. The weight of its brain is one-fourteenth of the entire weight of the body. The most unintelligent of all is the domestic fowl, whose body is 412 times heavier than its brain. The size of brain, however, seems to be largely a matter of the size of the bird: generally speaking, the smaller birds have heavier brains, and vice versâ. One might have expected something from the apparently intelligent Parrot; but the brain of the ‘Amazon’ is only one forty-second part of the weight of its body. Even the cruel and bloodthirsty Hawk, which one associates with brutality and ignorance, has a brain which is but little heavier.

The front part of the organ, known as the cerebral hemispheres, or, more briefly, as the cerebrum, is that part of the brain which is associated with intelligence. Now among the mammals this part of the brain is generally much furrowed, the brain surface being, therefore, increased without any actual increase in the skull-space required. This furrowing is met with in most mammals, but not always in the smaller and in the less intelligent kinds. But in the bird’s brain there are no convolutions: the surface is as smooth as in the reptile. Not even in the artful Raven, which some hold as the most highly developed of birds, is there a trace of the furrowing which one rightly associates, so far as the mammalia are concerned, with a high position in the series. The hinder part of the brain is known as the cerebellum; between this and the cerebrum are the optic lobes, of which there are only two, the mammals having four. From the brain arises the spinal cord, or marrow, which runs in the canal formed by the vertebræ, just as the brain lies in the brain-case. The nerves of the body come off either from the brain or the marrow, but it is not important to enumerate them. They show no difference in different kinds of birds.

The Muscles.

The muscles of a bird are what is popularly known as its flesh. When the skin is removed, the bones are seen to be covered by a mass of this flesh, which is of a red colour, darker in some birds than in others. For instance, in a Duck the colour is a dark red; in a Pigeon, quite a pale brown. The flesh is not, however, merely a thick sheet covering the bones: it can be separated into layers which are themselves made up of a number of separate pieces of muscle. These individual muscles are very commonly of a spindle-like shape, being thickest in the middle and dwindling towards both ends, where they often end in a tough substance called the tendon, which has a glistening and very characteristic appearance. All muscles are not of this form--sometimes they are strap-shaped; and not all of them end in tendons. As the most important act of the bird’s life that depends upon its muscles is flying, it is not surprising to find that the muscle which effects the downward stroke of the wing is the largest. This muscle is known as the great pectoral, and it is said to be almost as large as all the other muscles of the body put together. The way in which a muscle effects the movements of the bones to which it is attached is by contracting. All muscles are able to contract; they shorten, and, accordingly, the ends, with whatever they happen to be attached to are brought closer together. The contraction is governed by the nerves, and it has been discovered that the nerves actually end in communication with the fibres of which the muscle is composed. This pectoral muscle lies on the breast-bone, and nearly completely covers it; indeed, only the edge of the keel appears, and a very little tract at the sides. When this muscle is dissected away another muscle, not nearly so large, comes into view underneath it; this is called the pectoralis secundus, or the second pectoral. Its action is precisely the reverse of that of the great pectoral: it pulls the wing up instead of down. Between them, these two muscles do most of the work in flying. Naturally, in the ostrich tribe, which do not fly, they are much reduced in bulk. But they are never absent altogether, even in the Apteryx, which is, perhaps, further removed from the possibilities of flight than any other bird.

A very curious muscle runs into the patagium of the wing, which is that fold of skin which lies between the shoulder and the hand. This muscle is called the patagial muscle. It starts from the shoulder as a fleshy band, but soon ends in two long tendons: one of these follows the upper margin of the patagium, and finally ends in the wrist; the other passes down over the patagium, and ends below in connection with some of the muscles of the arm, and also by being attached in a fan-shaped way to the skin itself. The function of this muscle is to assist in the folding up of the wing when it is, so to speak, put away after use. The tendons in which the latter part of this muscle ends often show a most complicated branching in the patagium; they frequently offer characteristic differences in different birds, and are made some use of by the systematist. The bird has got a biceps to its arm just as we have. It sometimes happens that this biceps gives off a muscular slip, which runs into the patagium and becomes attached to the upper of the two tendons of the patagial muscle. A good deal of stress is laid by certain ornithologists as to whether this biceps slip is absent or present. Several of the common British birds will afford material to the beginner to ascertain for himself some of the chief variations in these and the other muscles of the body. It will be a good exercise to get a few birds, and to carefully dissect two of them, belonging to as widely different kinds as possible, side by side. You might select, for instance, a Crow and a Pigeon, which are fairly extreme types. To revert to our account of the muscular anatomy of a bird, it will be impossible to attempt any comprehensive account of this branch of the subject, because the facts are so appallingly numerous. We shall content ourselves, therefore, with the mention of a highly characteristic bird muscle which occurs in the leg. This muscle is known as the ambiens. This muscle is thin and ribbon-like. It takes its origin from a little process of the pubic bone usually called the prepubic process. From this point it runs along the inside of the thigh until it reaches the knee; it then bends over the knee and comes out on the other side, where it runs down the leg to join the deep flexor muscle of the foot. When this ambiens muscle contracts it pulls upon the flexor muscle, already referred to; the effect of this is that the toes are brought together by the tendons in which the last-mentioned muscle ends. The ambiens is far from being universally present among birds. It is notably absent from the passerine birds (the Sparrows, Crows, Rooks, and small perching birds generally), and from the Hornbills, Toucans, Woodpeckers, and that varied assemblage known as picarian birds. On the other hand, the Storks, Hawks, and most of the larger birds, have the muscle. But among some of these it is absent; thus, the Owls on the one hand, and the Herons on the other, have no ambiens; but from their general resemblance in other particulars to birds which have an ambiens, it was thought by Professor Garrod that the loss in them was a recent event, and that they might be fairly placed in one great group of birds with an ambiens which he termed, somewhat lengthily, the ‘homalogonatæ,’ or normal-kneed birds, reserving the name ‘anomalogonatæ,’ or abnormal-kneed birds, for the passerines, &c., without an ambiens.