70 DESCRIPTIONS OF PREPARATIONS. 



Circulatory organs. There is a sinus venosus formed by the union of the 

 vena cava inferior and the right jugular vein. It opens into the right auricle by 

 an aperture guarded by two valves. The left jugular opens into the auricle 

 separately. A single pulmonary vein opens into the left auricle. The two auricles 

 are separated by a septum, the free edge of which is produced into a right and 

 left auriculo-ventricular valve. The ventricle has a single cavity partially sub- 

 divided by a muscular band or septum on its anterior wall. To the right of this 

 septum is the cavum pulmonale from which the pulmonary artery arises. The left 

 side of the ventricular cavity is divisible in turn into a left cavum arteriosum into 

 which the left auricle opens and a right cavum venosum from which arise to the 

 right the left aorta, to the left the right aorta. Hence these two vessels cross at 

 their origins. In contraction of the heart the septum isolates the cavum pulmonale 

 completely. The three great vessels, i. e. two aortae and pulmonary artery have, 

 as in all Reptilia, two semilunar valves at their origin. 



In the arterial system the right aorta gives off, first, two coronary arteries ; 

 secondly, an arteria cephalica, the common origin of the two carotids ; thirdly, 

 an arteria collaris, which runs beneath the back-bone and ends close to the head. 

 The left aorta gives off no branches. The subvertebral aorta has no caeliac 

 axis ; there are several hepatic and renal arteries, and the right and left intercostal 

 arteries arise by a common stem. The aorta is continued into the tail as the 

 caudal artery. The pulmonary vein lies hidden by the vena cava. 



In the venous system, the right inferior jugular close to the heart receives two 

 veins, a short anterior and a long posterior azygos. The vena cava inferior is formed 

 by the union of the two efferent renal veins ; it receives the ovarian (or testicular) 

 veins, and the hepatic veins in its course along the liver. The portal vein rises on 

 the dorsal wall of the cloaca and receives the veins of the intestines, stomach, spleen, 

 pancreas, the epigastric vein and the intercostal veins. It runs on the visceral 

 surface of the liver to which it is distributed. The reni-portal veins of the kidneys 

 are formed by the bifurcation of the caudal vein. There are anastomoses between 

 them, the portal, and the epigastric veins. 



Digestive system. The tongue is partially contained in a sheath which opens 

 anteriorly on the floor of the mouth. Leydig describes a paired and an azygos 

 gland in connection with this sheath. The stomach gradually contracts to the 

 pylorus, which is well developed. The last six inches of the small intestine are 

 nearly straight ; it opens laterally into the large intestine, and there is in some 

 specimens a short caecum (as in the Pythons) at this spot. The large intestine 

 is about three and a half inches long ; it gradually increases in calibre to the 

 cloaca, from which it is marked off by a constriction. 



Respiratory system. The posterior portion of the lung is thin and not alveo- 

 lated, and receives blood from the hepatic arteries. In Pythons the left lung is 

 functional, though smaller than the right. In Hydrophis cyanocincta a marine 

 serpent the lung extends to the cloaca, beneath the back-bone. 



Urinary system. The ureter runs centrally between the reni-portal vein on the 

 outer and the efferent renal vein on the inner side. There is no urinary bladder. 



Generative organs. The testes are elongate, rounded organs with a small 

 epididymis, and placed asymmetrically like the ovaries. The vas deferens is 

 thrown into short coils and accompanies the ureter of its own side, and opens 



