VII.] THE SEMILUNAR VALVES. 191 



ascertained at this stage by dissection or by sections, there 

 being as yet no external signs of the division. 



At the time when the septum is first formed, the opening 

 of the bulbus arteriosus into the ventricles is narrow or slit- 

 like, apparently in order to prevent the flow of the blood 

 back into the heart. Soon after the appearance of the septum, 

 however, semilunar valves (Tonge, loc. cit.) are developed 

 from the wall of that portion of the bulbus which lies between 

 the free edge of the septum and the cavity of the ventricles. 



These arise as six solid outgrowths of the wall arranged in 

 pairs, an anterior, an inner, and an outer pair, one valve of 

 each pair belonging to the one and the other to the other of 

 the two main divisions of the bulbus which are now being 

 established. 



The anterior and the inner 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 

 shallow ridge, in the centre of which is a prominence indi- 

 cating the point where the ridge will subsequently become 

 divided into two. The outer pair of valves appear opposite 

 each other, at a considerably later period, between the ends 

 of the other pair of valves on each side. 



As the septum grows downwards towards the heart, it 

 finally reaches the position of these valves. One of its legs 

 then passes between the two anterior pair of valves, and the 

 other unites with the prominence on the inner 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 bulbus now fuses with the ventricular 

 septum, and thus the division of the bulbus 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 what 

 it is in the adult heart. 



That division of the bulbus 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. 



