Part 96

The first dissection should be to lay bare the dermal muscles. The dermal muscles are of two kinds, true dermal and dermo-osseous. The dermal muscles have their origin and insertion in the skin, and control the movements of the different groups of feathers. The dermo-osseous have their origin on some part of the skeleton, and insert to the integuments.

The dermal muscles vary with the characteristics of the bird, we do not find all the known dermal muscles in any one specimen. A cock of the Cornish breed will show these muscles best developed. Birds possess an enormous system of minute muscles divided up into an infinite number of fasciculi, to act harmoniously upon the feather quills and to agitate collectively the plumage. By the aid of a low-power lens the action of the feather muscles in the large quill butts of the wing or the tail may be studied.

The muscles may be studied in groups as outlined in the text. Make an incision through the skin down to the bone on the superior part of the head, parallel and close to the base of the upper mandible, and extending completely across. From the outer end of this make an incision backward and down to the skull and posteriorly. The muscles of the upper part of the neck will then be exposed. The straight incision should extend to about a half inch on the inside of the upper eyelid of the same side. Reflect the flap of skin from the top of the skull, and carefully examine the under side of it in the median line, where it overlies the frontal region. The dermo-frontalis will be observed if it be present. In many birds, especially in females, it may not be discernible, and may be considered absent. To expose the circumconcha make an incision completely around the ear; then carefully dissect to the ear base. A dermal circular muscle should be observed. To expose the dermo-temporalis extend the longitudinal incision down the back of the neck to a point between the clavicular heads, carrying it just through the skin and about one-fourth of an inch to the side of the median line. Remove the skin from the throat and the anterior portion of the chest. Lay open the alar and parapatagial duplicatures of the skin. This exposes a number of dermal muscles. The dermo-temporalis is now observed to extend from a small depression just above and anterior to the temporal fossa. It makes slight attachments to the temporal muscle, which it covers, and extends backward as a thin ribbon-shaped muscle, the fibers blending with those of the cleido-trachealis, and becomes lost upon the skin in front and opposite the shoulder-joint. At times its fibers blend with those of the dermo-tensor patagii.

As the musculature of the fowl is loosely arranged, the rest of the dissection is easily done if care be exercised.

The ligaments may be dissected after the completion of the study of muscles, using the same subject or, a two pound broiler be prepared by killing in the gas chamber, plucking the feathers and parboiling just till the flesh becomes tender and is easily removed, it will be observed that all structures can be removed from the points exposing the ligaments distinctly. The ligaments appear swollen and more easily observed for study.

=Directions for the Study of the Viscera.=—Carefully remove the right and the left abdominal and thoracic walls, allowing a strip of tissue to remain in the median line to hold the organs in their normal position. To open these cavities it is necessary to use the bone saw and the scalpel. The organs may now be studied from each side. To make a longitudinal section through the median line, select a small bird, one weighing not more than 2 pounds, kill, and preserve in a 10 per cent. solution of formaldehyde for three days. Then with a sharp, thin, long-bladed knife make an incision at one sweep through the median line of the body down to the back bone, and with the bone saw section through the vertebræ. If it is difficult to cut through the breast-bone, saw through before making the incision. In small birds the entire cut may be made without the aid of the saw.

=Directions for the Study of Arteries.=—Arteries should be injected. Veins are usually more or less filled with blood so that the tracing of these is not so difficult as the tracing of uninjected arteries. Nerves are white and no difficulty is usually encountered in tracing them.

The courses of arteries, veins, and nerves are side by side, and many of them, as in mammals, are arranged in the order of veins, arteries, nerves, the veins being in front.

The injection apparatus consists of the following parts: air-compression chamber, to which is attached a pressure pump. A manometer made of glass tubing 6 millimeters in diameter, inside measurement. This tube is partly filled with mercury, and a scale in centimeters is made from the top of the right-hand tube (Fig. 84, No. 8). This ruling, or gauge, should be about 15 centimeters long. Extending from the right extremity of the U-tube is a small rubber tubing which is attached to the chamber containing the injection fluid; and extending from the inferior part of this chamber is another tubing which has the injection needle attached to the free end.

The injection should be done under 120 millimeters pressure. The stop cock of the pressure chamber is released sufficiently to raise the mercury in the U-tube six centimeters, which multiplied by 2, the amount of work required to raise two columns, makes 120 millimeters pressure. If this pressure be maintained, all vessels should be injected without rupture. The same process may be used in injecting the air cells through the trachea.

The injecting material may consist of one part finely sifted plaster of Paris, four parts water, and sufficient gentian violet to make a violet color. For the coloring, red aniline may be used in preparing this material. The dye should be dissolved in the water to be used in making the injection liquid. Caution must be used and the work rapidly done, as the plaster soon sets, or becomes solid, in the needle or in the tubing. A small cannula should be used, since the endothelial lining of the arteries are easily injured and difficulty may thus be created.

Select for arterial dissection an old cock, as in a bird of this kind the arteries are larger and the difficulties are reduced. Select for bleeding and injection the ischiadic artery in the thigh region. With the sharp point of the thin blade of a knife make an incision lengthwise of the artery being careful not to strip back the endothelial lining of the artery. Allow as much of the blood as will escape before injecting; in fact, the arteries should be thoroughly emptied, so that there is no longer danger of a clot’s plugging some vessel and thus preventing its filling. Since the blood of most birds coagulates in about thirty seconds, this work must be done rapidly, care being exercised to keep the flow running as long as possible. After bleeding is completed, insert the cannula and tie the vessel tightly around the cannula to prevent the escape of the injecting fluid. See that all connections are sufficiently tight to prevent the escape of liquid under pressure. After the injection is completed, remove the cannula and tie the artery with a small twine, preferably cotton. Quickly remove all injecting fluid from the needle, the tubing, and the injecting chamber.

During the operation of bleeding the cock may be chloroformed, care being taken not to administer chloroform to kill him; for it is necessary to maintain life as long as possible so that the heart may be kept beating and all blood possible drained from the arteries. After the injection is completed the bird may be plucked and immersed in the preservative fluid in the same manner as in the preparation for dissection for muscles.

The arteries, the veins, and the nerves may now be dissected and studied in relation to one another and in relation to the muscles, the bones, and other structures. The skin should not be removed from the shanks till it is desired to dissect these parts, as the tissues quickly dry out. In fact, the tendons of the shanks and toes can best be dissected while the specimen is fresh.

=A Study of the Structure of Bones.=—Longitudinal and transverse sections of old bone may be made by making thin longitudinal and transverse sections with the bone saw, and then by making them very thin with a fine three-cornered file. Examining under the low power microscope, we note the lacunæ, the canaliculi, and the Haversian system.

Similar sections in green bone may be studied if prepared as follows. Secure specimen of bone just removed from a fowl and place it for three days in a 10 per cent. aqueous solution of hydrochloric acid. Test by puncturing with a needle, and, if all the mineral salts are removed, place in a water bath and wash for four hours. Pass it through the fluids usually employed in preparing specimens for sectioning with the microtome. Stain as sections of other tissue for microscopic study. See the description below. If the ends of the bone be included, it will enable the student to study not only compact bone but also cancellated bone and articular, or hyaline cartilage, and in some of the bones, as the femur, the red marrow.

=Special Technic for the Dissection of Cranial and Spinal Nerves.=—It is rather difficult to dissect the cranial and spinal nerves of the fowl, owing to the fact that the structures are very small. The bone is rather hard and the nerve tissue so delicate that great skill must be attained to achieve any degree of success.

A simple technic has been developed as follows: Place the head and neck, or other structures of the spinal column in a 10 per cent. aqueous solution of hydrochloric acid for three or more days, the time depending on the size of the specimen and the amount of soft structures surrounding it. This solution removes all the calcium salts from the bone and makes the removal of the bony structures a less difficult task.

=Directions for the Study of Soft Structures.=—Secure a specimen of the tissue to be studied—lung, muscle, intestine, liver, pancreas—from a normal fowl just killed. The specimen should be not more than ½ inch square. After first hardening three days in 10 per cent. formaldehyde. Pass through the following fluids:

1. Alcohol, 95 per cent. 24 hours 2. Alcohol, absolute 24 hours 3. Alcohol and ether, equal parts 24 hours 4. 1 per cent. celloidin 24 hours 5. 2 per cent. celloidin 24 hours 6. 4 per cent. celloidin 24 hours 7. 6 per cent. celloidin 24 hours 8. 10 per cent. celloidin 24 hours 9. Place on block, and as soon as solid, place in 80 per cent. alcohol until ready to section. Histoloid or parlodion will take the place of celloidin.

In placing specimen on the block be careful that the specimen lies conveniently for cutting the sections in the right direction. As soon as the surface has hardened a little, add a few drops of thick celloidin, and repeat until there is a good body of celloidin. Allow to stand until the tissues are quite firmly fastened to the block, but not long enough to permit shrinking. Then place in 80 per cent. alcohol until the specimen is perfectly firm, 12 or more hours, before cutting.

All tissues, cut sections, and mounted blocks are to be placed in 80 per cent. alcohol. As containers for this purpose shell vials will be most handy.

Cut the sections with the microtome as thin as possible, the thinner the better. The following process of staining will make the nucleus blue and the cytoplasm reddish.

1. Float section in a tumbler of tap water.

2. Place section on slide, and immerse in hematoxylon for five to ten minutes.

3. Immerse in acid alcohol from two to five seconds.

5. Place on slide, and immerse in eosin from one-half to three minutes.

6. Wash thoroughly in alcohol.