A COMPARATIVE STUDY OF THE CIRCULATORY SYSTEM 257 



is returned from the system, flows into the right auricle, while the 



blood which has just been aerated in the lungs, enters the left auricle. 



When these parts contract, both types of blood are simultaneously 



forced into the ventricular cavity, where they must intermingle 



somewhat, because they are not kept apart by partitions. It must be 



emphasized, however, that a thorough mixture of the aerated with the 



venous blood cannot take place, because the interval between the 



auricular and ventricular contractions is extremely brief, and because 



the ventricular wall contains numerous recesses, in which at least a 



part of the venous and oxygenated types of blood finds separate lodg- 



ment. It is only natural to suppose that these types of blood will be 



forced into those parts of the ventricle 



which lie directly below their respective 



auricular orifices. It is also true that 



the venous blood reaches the conus 



arteriosus ahead of the oxygenated, be- 



cause the right expanse of the ventricular 



cavity lies nearest this structure. More- 



over, as the resistance in the pulmonary 



circuit is less than that in the systemic 



blood-vessels, the first gush of ventricular 



blood, venous in character, must find its 



way into the lungs by way of the pulmo- 



nary artery, while the aerated portion 



following it must necessarily be diverted 



into the peripheral channels. A special 



system of blood-vessels for the muscula- 



ture of the heart is not present in amphi- 



bians. These animals, however, are in Fro. 126. DIAGRAM TO SHOW 



possession of a hepatic portal system and E COURSE OF THE BLOOD 



THROUGH THE REPTILIAN HEART. 



a peculiar renal portal system. The 



latter modification of the vascular mechan- ?Y' 1T 8 i v ,? nosu . s : , R *i right 



, . . . , , , , , , auricle; LA, left auncle; V, ven- 



ism finds its Origin in the double blood- tricle incompletely divided by a 



supply of the amphibian kidney. Itwill be septum; A, aorta; PA, pulmo- 

 remembered that its glomeruli receive their ^. ^fStS-jS^S. 

 blood from the aorta directly, while the re- tains venous blood, the non- 

 maining portions of the urinary tubules striated arterial blood. 

 are supplied by the renal portal vein. 



The heart of the reptiles resembles that of the amphibians in 

 several particulars. It also consists of a sinus, two auricles, and a 

 ventricle. A two-lipped valve is situated in the sino-auricular orifice 

 and a right and left semilunar valve in the corresponding auriculo- 

 ventricular openings. The ventricle, from which the aorta and pul- 

 monary artery emerge separately, is divided into two compartments 

 by a muscular septum. The separation is complete in the crocodiles, 

 but incomplete in the snakes, lizards, and turtles. In the animals 

 named last, the tendency is to keep the venous blood separated from 



17 



