STRUCTURE OF THE VERTEBRATES 229 



treated in connection with the comparative study of the differ- 

 ent regions of the system, and the developmental changes will be 

 discussed in connection with the comparative anatomy of the 

 vessels. 



C. Development of the Vascular System 



The student at this time should have a concept of the general 

 structure of the vascular system. It is clear that the entire group 

 of structures included in the system is a unified whole, both 

 anatomically and functionally. For convenience the system can 

 be divided into its integral parts and each treated separately. 

 It should be borne in mind that development is synchronous, the 

 entire system growing and developing as a unit in correlation 

 with the growth and differentiation of the other organs. 



1. Comparative Anatomy of the Heart. Amphioxus has a 

 straight, muscular, pulsating ventral aorta. In the cyclostomes 

 the heart is S-shaped, with a single dorsal auricle and a heavy- 

 walled ventricle. The blood from the systemic veins empties 

 into a sinus venosus, from which it passes to the auricle, then 

 to the ventricle and is forced into the ventral aorta. In the larva 

 of the lamprey (page 31) the coelomic cavity surrounding the 

 heart is partly cut off from the body coelom, and in the adult the 

 separation is completed. This coelomic pocket is the pericardial 

 cavity and is found in all the vertebrates. 



The dogfish heart is essentially similar, with the addition of a 

 conns arteriosus containing numerous cup-shaped semi-lunar 

 valves to prevent the back flow of the blood from the ventral 

 aorta. This muscular conus is gradually lost in the higher fish 

 and the semi-lunar valves reduced to a typical three. 



The amphibia make a definite advance in heart structure with 

 the separation of the auricle into two distinct chambers, the 

 dividing septum passing between the pulmonary veins which 

 enter the auricle directly, and the sinus venosus which receives 

 the body veins. A single auriculo-ventricular opening is present. 

 The left auricle consequently contains aerated blood, and the 

 right is filled with unaerated; but there is necessarily an ad- 

 mixture of blood as it enters the single ventricle. In this respect 

 the amphibian heart is less efficient than that of the fish. The 



