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 

 possession of a hepatic portal system and 

 a peculiar renal portal system. The 

 latter modification of the vascular mechan- 

 ism finds its origin in the double blood- 

 supply of the amphibian kidney. It will be 

 remembered that its glomeruli receive their 



Fig. 126. — Diagram to Show 

 THE Course op the Blood 

 Through the Reptilian Heart. 



SV, sinus venosus; RA, right 

 auricle; LA, left auricle; V, ven- 

 tricle incompletely divided by a 

 septum; A, aorta; PA, pulmo- 

 nary arteries; PV, pulmonary 

 vein. The striated portion con- 



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 



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