Elementary Zoology, Second Edition

by Vernon L. Kellogg

Make a drawing of the dissection as so far worked out.

Technical Note.—Remove the alimentary canal, cutting it free posteriorly at the cæca and anteriorly just above the muscular gizzard. Cut open the gizzard and note its structure. The contained sand and gravel grains are picked up by the bird as it eats.

On either side of the throat note the well-defined thyroid gland; in young sparrows will be noted on each side of the neck a mass of tissue, the remains of the thymus gland, which disappears in the adult.

Cut transversely through the lower end of the heart and note that the ventricles are wholly distinct, whereas in the toad and snake they are incompletely separated. In the bird there is a complete double circulation. Its blood is not mixed, the pure with the impure, as in the toad and snake. Blood passing through the right auricle and ventricle goes to the lungs; on its return to the heart purified, it enters the left auricle and left ventricle thence to pass out over the body through the arteries.

Note the large aorta given off from the left ventricle. Note the two large branches, the innominate arteries, given off by it near its origin. Each innominate divides into three smaller arteries, a carotid, branchial, and pectoral. The aorta itself turns toward the back and continues posteriorly through the body as the dorsal aorta. To the right auricle come three large veins, the right and left præcavæ and the postcava. Each præcava is formed by three veins, the jugular from the head, the branchial from the wing, and the pectoral from the pectoral muscles. The postcava comes from the liver. From the right ventricle go the short right and left pulmonary arteries to the lungs, and from the lungs the blood is brought to the left auricle through the right and left pulmonary veins.

Technical Note.—For a detailed study of the circulation of the bird the teacher should inject the blood system of some larger bird, as a pigeon or fowl, for a class-demonstration. (For a guide, use Parker's "Zootomy," p. 209, or Martin and Moale's "How to Dissect a Bird," pp. 135-140 and pp. 148, 149.)

In the posterior dorsal region of the body-cavity will be found large three-lobed organs fitting into the spaces between the bones of the back on either side. These are the kidneys, and from their outer margins on each side a ureter runs posteriorly into the cloaca. Overlying the anterior ends of the kidneys are the reproductive organs. In the male these glands consist of firm, whitish, glandular bodies. From each runs a long convoluted vas deferens, which enters the cloaca. This tube corresponds to the egg-duct of the female. In the female the right egg-gland and egg-duct or oviduct are wanting. The left egg-gland appears as a glandular mass; during the breeding season yellow ova or eggs in various stages of development project from its surface. The oviduct opens by a funnel-shaped mouth near the egg-gland and runs thence to the cloaca. The eggs pass from the egg-gland into the body-cavity, where they are caught in the upper end of the oviduct and carried down and out through the cloacal opening. It is in the oviduct that the egg derives its accessory covering, which consists of a white or albuminous portion, together with several enveloping membranes and the hard shell enclosing all.

Remove the top of the skull and note the large brain. What portions of the brain make up the greater part of it? Note the differences between this brain and that of the toad. Trace the principal cranial nerves. Work out the spinal cord and principal spinal nerves. For an account of the nervous system of the sparrow see Martin and Moale's "How to Dissect a Bird," pp. 150-163.

Technical Note.—For a study of the skeleton of the sparrow a specimen should be cleaned by boiling in a soap-solution (see p. [452]).

In the sparrow's skeleton note the compactness of the skull and the fusion of its bones. Observe the long cervical vertebræ which support the skull, also the thoracic vertebræ bearing the ribs and sternum. How many of each of these kinds of vertebræ are there? The vertebræ posterior to the thorax are more or less fused together to form the sacrum, which, with the pelvic girdle, supports the leg-bones. The bones of the tail consist of a number of very small vertebræ, some of which are fused together. Note the correspondence between the bones of the leg and those of the wing. What are the names of each of the bones of each limb, and what are the corresponding bones in the two limbs? The wings and legs being modified for different uses, their various bones have assumed different relations to each other and to the body, for they are bent at directly opposite angles and the attachment of muscles is different. Compare the skeleton of the bird with that of the toad. (For a detailed account of the skeleton of the bird see Parker's "Zootomy," pp. 182-209, or Martin and Moale's "How to Dissect a Bird," pp. 102-125.)