584 COMPARATIVE ANATOMY. 



larger space (foramen ovale). Owing to this connection^ the blood 

 which passes into the right ventricle from the umbilical vein through 

 the vena cava inferior, is able to pass into the left ventricle, whence 

 it is distributed through the body by the aorta. In the Mono- 

 delphia this opening is gradually closed by the forward growth of a 

 wall, or partition, towards the left auricle (valvula foraminis ovalis) ; 

 after birth, therefore, the two auricles are completely separated. 

 The boundary of the primitive foramen ovale may, even later, be seen 

 as a circular process. The most anterior (ventral) portion of each 

 of the two auricles in the Mammalia forms a small prolongation, 

 which has a different form on either side, the "appendages of 

 the auricles." They correspond to the greater part of the auricles 

 of the lower classes, for the hinder portion of the cavity of the 

 auricles, on the right side, at any rate, is formed by a venous sinus, 

 which in the lower forms is separated from the auricle (cf. Venous 

 System). These appendages, therefore, in the Mainmalia are the 

 remnants of the antei'ior portion of the auricle. 



The atrio-ventricular valves are much altered in character. In 

 the earliest stages their place is taken by membranous folds, as in 

 Fishes, Amphibia, and Eeptilia. The ventricles have a compara- 

 tively small cavity, and their wall is formed by the same spongy 

 muscular tissue as that which we have found in a well-developed con- 

 dition in the above-mentioned classes. The bands gradually thicken, 

 and part goes to form the more compact cardiac wall. That portion 

 of this network which passes inwards and bounds the lumen of the 

 ventricular cavity, and which is inserted into the edges of the 

 venous ostium, is connected with these membranous valves. As 

 the fi-ee edge of the valve ceases to grow, and as the only part 

 which is left is that which is connected with the muscular bands, the 

 valve becomes intimately connected with the wall of the ventricle, 

 so that the muscular bands from the latter pass into a membrane 

 which is given off from the ostium. This condition, which is 

 transitory in most Mammals, is permanent in the right ventricle of 

 the Monotremata (Ornithorhynchus). The muscular bands which pass 

 oif from the wall of the ventricle are continued into a membranous 

 valve. In all other Mammalia this condition is replaced by another 

 one. _ The muscular bands are withdrawn towards the wall of the 

 ventricle, and so form the musculi papillares, while their more 

 anterior portion, which passes into the valve, is represented by the 

 chordge tendine^e. The rest of the spongy muscular network of the 

 wall of the ventricle forms the trabecule carnege. The atrio-ven- 

 tricular valves and the chordte tendinese are, therefore, differentia- 

 tions of the primitive muscular network, which was connected 

 with the primitive valves. The similar valves in the left ven- 

 tricle of the Bird's heart are formed in just the same way. The 

 great arterial trunks in the Mammalia are differentiated from 

 the embryonic arrangements in a somewhat different manner. The 

 two first arches disappear completely ; the third, as before, forms 

 part of the carotid. The fourth is continuous on the right side with 



