DEVELOPMENT OF THE CIRCULATORY SYSTEM. 



241 



that the interior of the ventricular cavities during the second 

 month is filled with a muscular sponge work, like the ventricle 

 of the frog's heart (Fig. 199). Out of this muscular network 

 are formed : 



(1) Chordae tendineae ; 



(2) Musculi papillares ; 



(3) The moderator band ; 



(4) Trabeculae and columnae carneae; 



(5) Musculi pectinati (in the auricular appendages). 



right aur-vent. orif. 

 tricuspid 



left aur-vent. 

 ' orifice 



bicuspid value 



muscular , 

 sponge-work 



intervent. sept. 



Fig. 199. — Section of the Ventricles of the Foetal Heart, showing the Muscular 

 Sponge Work within their Cavities. (After His.) 



The endocardium lining the auricular canal (see Figs. 197 

 and 199) becomes folded or protruded within the ventricles by 

 a shortening of the auricular canal. The funnel-shaped fold or in- 

 flection of endocardium is the basis of the auriculo-ventricular valves. 

 They are formed at the same time as the septa of the heart. 

 Originally funnel-shaped, the left or mitral comes to show two 

 cusps — septal and left, the right or tricuspid three segments — 

 septal, anterior and posterior. The funnel shape is frequently 

 partially retained. The cardiac tissue on the outer aspect of 

 the valves is opened out into a sponge-work continuous with that 

 of the ventricle. Hence the attachment of the chordae tendinae 

 on the outer surfaces and edges of the valves. 



The Semilunar Valves. — The bulbus arteriosus is divided by 

 the aortic septa into an anterior and right part — the pulmonary 

 aorta, and a posterior and left — the ascending aorta. Just 



Q 



