THE VASCULAR SYSTEM. 



641 



The ventral and the dorsal pairs of valves are the first to appear : the 

 former as two small solid prominences separated from each other by a 

 narrow groove ; the latter as a single ridge, in the centre of which is a 

 prominence indicating the point where the ridge will subsequently become 

 divided into two. The outer valves appear opposite each other, at a 

 considerably later period. 



As the septum grows downwards towards the heart, it finally reaches 

 the position of these valves. One of its edges then passes between the two 

 ventral valves, and the other unites with the prominence on the dorsal 

 valve-ridge. At the same time the growth of all the parts causes the valves 

 to appear to approach the heart, and thus to be placed quite at the top 

 of the ventricular cavities. The free edge of the septum of the truncus now 



A. 



B. 



r.v 



la. 



r.a 



FlG. 3(1 1. TWO VIEWS OF THE HEART OF A CHICK UPON THE FIFTH DAY 



OF INCUBATION. 



A. from the ventral, B. from the dorsal side. 



/.<?. left auricular appendage; r.a. right auricular appendage; >:?'. right ventricle; 

 /.?'. left ventricle; f>. truncus arteriosus. 



fuses with the ventricular septum, and thus the division of the truncus into 

 two separate channels, each provided with three valves, and each com- 

 municating with a separate side of the heart, is complete ; the position of 

 the valves not being very different from that in the adult heart. 



That division of the truncus which opens into the fifth pair of arches is 

 the one which communicates with the right ventricle, while that which 

 opens into the third and fourth pairs communicates with the left ventricle. 

 The former becomes the pulmonary artery, the latter the commencement of 

 the systemic aorta. 



The external constriction actually dividing the truncus into two vessels 

 does not begin to appear till the septum has extended some way back 

 towards the heart. 



The semilunar valves become pocketed at a period considerably later 

 than their first formation (from the I47th to the i65th hour) in the order of 

 their appearance. 



At the end of the sixth day, and even on the fifth day (figs. 361 and 362), 

 the appearance of the heart itself, without reference to the vessels which 

 come from it, is not very dissimilar from that of the adult. The original 



B. III. 



