Elementary Zoology, Second Edition

by Vernon L. Kellogg

When the snake is laid open the elongate heart will be conspicuous in the anterior third of the body. Insert a blowpipe or quill into the glottis just back of the tongue, and inflate the lung, which is a long, thin-walled bag extending from the region of the heart posteriorly for two-thirds of the length of the body. There is but one developed lung, the right; note at the anterior end of the lung a small mass of tissue, the atrophied left lung. Running forward from the lung is a long tube composed of incomplete cartilaginous rings, connected by membrane, the trachea. Note the long straight alimentary canal. Distinguish the œsophagus, stomach, intestine, rectum and the anus.

In the region of the lung is an elongated dark-red glandular mass, the liver. The secretion from the liver passes down through the long hepatic duct to the oval-shaped green gall-bladder and into the intestine.

Technical Note.—The bile-duct may be injected through the gall-bladder with some colored injecting mass.

Note that the duct running off from the gall-bladder to the intestine passes through a pink glandular organ, the pancreas. At the anterior end of the pancreas is a dark-red nodular structure, the spleen. The alimentary canal, the liver and the spleen are all suspended from the dorsal wall of the body-cavity by a delicate sheet of tissue. What is this? This condition we have also noted in the toad and fish.

Toward the posterior end of the body cavity are two long, dark-red glands, the kidneys, which are the principal excretory organs of the body. Through a long, slender tube (the ureter) each of the kidneys passes off its wastes. Where do the ureters open?

Anterior to the kidneys are the reproductive organs. The eggs, produced by the female snake, after being fertilized, pass backward through the egg-tubes. During the breeding season these tubes are much distended. This is due to the presence of the developing eggs, for the young snakes are hatched in the egg-tubes.

A successful injection as directed in the first technical note will have filled both arterial and venous systems. How does the general shape of the snake's heart compare with that of the toad? The heart consists of two ventricles, incompletely separated, and two auricles. In the snake the conus arteriosus is very much shortened and is not visible. Note two large vessels arising from the median portion of the ventricle. The one on the left side is the left aortic artery or left aortic arch, while the right gives off two branches. Where does the anterior one of these run? The main branch, or right aortic arch, passes back to meet its fellow, the left aortic artery, forming with it the dorsal aorta, which runs posteriorly to the end of the tail. Note the various branches given off by the dorsal aorta and trace some of them. Arising from the ventricles beneath the two aortic arches is the pulmonary artery, which goes to the lung. There the blood is purified, after which it is taken up by the pulmonary vein and carried back to the left auricle, whence it passes into the ventricle to be mixed with the impure blood from the right auricle. From the arteries the blood flows to all parts of the body through fine capillaries, bathing the tissues, giving off oxygen and taking up the carbonic acid gas. From these capillaries it passes into veins and so back to the heart; from the anterior end of the body through the jugular veins and from the posterior portion of the body through the postcaval vein. Flowing forward from the tail in the caudal vein, the blood enters the capillaries of the kidneys, where the waste matter is taken from it. This part of the circulatory system is known as the renal-portal circulation. From the kidneys the blood flows through the postcaval vein anteriorly to the heart.

The blood which passes out from the dorsal aorta to all parts of the alimentary canal is again collected into veins which unite to form the mesenteric vein. This vein runs to the liver, where it breaks up into capillaries. Thence the blood is carried into the postcaval vein, which leads directly to the heart. This part of the circulatory system which collects blood from the alimentary canal and carries it to the liver is called the hepatic-portal system.

Just in front of the heart will be noted a nodular structure, the thyroid gland, while a little in advance of the thyroid may be seen a long glandular mass, the thymus gland. The functions of these glands are not certainly understood.

Remove the alimentary canal and muscles from a part of the body and note that the axial skeleton, like that of the other vertebrates studied, consists of a series of vertebræ placed end to end. Are there arms or legs? Are shoulder and pelvic girdles present? How many of the vertebræ bear ribs? The ribs connect at their lower ends with the ventral scales. Note the great number of the vertebræ and ribs as compared with those of the toad or fish. What are those vertebræ called which bear no appendages or ribs? Examine carefully the elongated skull of the snake, especially the modified jaws. A detailed study of the skeleton may be made by referring to the account of the skeleton of the lizard in Parker's "Zootomy," pp. 130 et seq.