CROCODILIANS, LIZARDS, AND SNAKES. 693 



poial muscle extends over the poison glaud and compresses it when 

 the mouth is opened. In the venomous groups the anterior glands are 

 reduced. Besides the labial glands there is a gland anterior to the 

 prefrontal region, between the maxillary and nasal bones. The liver 

 is long and narrow, and is on the right side of the alimentary canal, 

 and begins immediately i)osterior to the heart or at various distances 

 posterior to it, according to the genus. Thus it is near the heart in 

 species of robust form, as the Viperidne and Crotalid;e, and in such 

 genera as Xenodon and Heterodon. It is far removed in the Tortricina, 

 Epanodouta, and Catodonta. It is exceptionally short in Chersydrns. 

 Its distal extremity is bilobate, and the left lobe extends beyond the 

 right. From between the two issues the cystic duct, which extends to 

 and along the alimentarj^ canal to the position of the gall bladder. This 

 stiucture is remarkable in Serpentesfor its distance from the liver. It 

 is near to the spleen, which is adherent to the alimentary canal on its 

 inferior side, and is of a subround form. The pancreas, which is of 

 more elongate form but of relatively small size, is located near the 

 spleen on the left side of the alimentary canal. 



THE CIRCULATORV SYSTEM. 



The disposition of the parts of the circulatory system in the Serpentes 

 is dependent in large degree on the elongate form of these animals 

 and on the arrangement of their respiratory organs. 



Although the heart is situated cephalad of the posttracheal lung, 

 its position is posterior to that which it occupies in the Sauria. In 

 CltcKsydnis it is situated at about the middle of the length of the body, 

 but in the Matrices it is at the anterior fifth of the length. It is of 

 rather elongate form, and its chambers are compactly adherent, and it 

 is inclosed in a tough pericardial sac. In adult snakes there is but one 

 aorta root on each side, of which the left is the most robust. The com- 

 mon pulmonary trunk is distinct from the truncus arteriosus to the base. 

 It was first observed by tSchlemm^ that in typical Colubroidea there 

 is but one pulmonary artery, while in the Peropoda there are two. He 

 also observed that where there is a tracheal lung there is an anterior 

 pulmonary artery, as in Vipera herns, while in Lachesis mutus there are 

 two. In Thrasops flarir/idaris, where the trachea is expanded trans- 

 versely to the size of a tracheal lung, I have observed that there is 

 an anterior pulmonary artery. There is generally a single carotid 

 arteiy, which arises from the right aorta root. This may branch into 

 two carotids, and in a few instances these carotids maintain their dis- 

 tinctness to the aorta root. The right aorta root also gives forth an 

 anterior artery, the arteria vertebral is of Ouvier, which supplies the 

 intercostal arteries. It extends along the right side of the vertebral 

 column, or divides, each half running on one side of the column. The 

 coeliac artery is divided into several. The mesenteric is represented 

 by two trunks which leave the aorta at a considerable distance apart. 



* Tiedemaun and Treviranus' Zeitschrift i. Physiologic, II, 1827, p. 101. 



