776 COMPARATIVE ANATOMY 



tract. This subintestinal vein forms a loop around the anal opening and 

 extends to the end of the tail as the caudal vein. The subclavian vein 

 from the arm may empty either into the jugulars or the post cardinal 

 near the Cuvierian duct. The blood from the hind limb leaves by an 

 iliac vein on each side, runs forward on the lateral body wall and is called 

 the lateral abdominal vein. This also enters the Cuvierian duct. 



Omphalomesenteric and subintestinal veins belong to the visceral 

 or splanchnic group. The others are somatic. The vessels mentioned 

 are important, and should be known thoroughly because they develop 

 very early in the embryo, and, practically all later developments as well 

 as modifications that take place in them, can only be discussed intelli- 

 gently when the basic structures just mentioned are known. There is 

 probably no more variable system in the body (even in the same species) 

 than the vascular. 



DETAILED STUDIES 

 THE HEART 



The heart itself is a muscle having a distinctive cellular structure, 

 this being a sort of "cross" between voluntary and involuntary muscle, 

 the muscle fibers are striated but run in a syncitial form (Fig. 23). 

 The muscular walls of the heart itself are known as myocardium. The 

 inner layer of the heart, corresponding to the endothelium of the blood 

 vessels and continuous with them, is called endocardium, while the cov- 

 ering of the heart is known as the pericardium. Lying between the 

 myocardium and the pericardium is a serous liquid called pericardial 

 fluid. 



We have already discussed a part of the embryonic method of heart 

 development, but it is necessary here to enter into more detail. The 

 lateral plates of the walls of the coelom grow centrally beneath the diges- 

 tive canal. There are four regions discernible in these lateral plates, 

 namely, the splanchnic or visceral, the mesenterial and somatic walls, 

 as well as the coelomic cavity. 



Between the coelomic walls and the endoderm one may observe vari- 

 ous cells called vascular cells. It is supposed that they find their origin 

 from the mesothelium. Those that lie most dorsalward assist in form- 

 ing the dorsal blood vessels, while those lying ventrad contribute to the 

 heart and the ventral trunks. The two lateral plates just mentioned con- 

 tinue until they meet in a ventrad region. This forms the ventral meso- 

 cardium (Fig. 344). A little later the dorsal region comes together form- 

 ing the dorsal mesocardium, so that now, that which was formerly a 

 groove has become a definite tube. The ventral fusion has disappeared, 

 leaving the dorsal part attached and causing the two coelomic cavities 

 to unite, forming the pericardial cavity. 



In turtles and crocodiles there is a small portion of this ventral 



