296 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



ceives the ducts of Cuvier and the omphalomesenteric veins. The 

 ventricle, also, does not reach the anterior wall of the pericardium, 

 but the anterior part of the heart tube forms a smaller trunk, the 

 tnincus arteriosus, while from the pericardium to the mandibular 

 arteries is an arterial vessel, the ventral aorta (fig. 320). 



Muscles, as stated above, are developed in the wall of the heart, 

 but to an unequal extent in the different parts, being scanty in the 

 sinus venosus, and most abundant in the ventricle where they form 

 an internal framework of trabeculae, giving the ventricle a somewhat 

 spongy characteristic. Folds or valves of the endocardium appear 

 in places at an early date and are so arranged that they permit the 



blood to flow foward but prevent any 

 backflow. In the base of the truncus 

 these valves take the form of pockets on 

 the walls, there being several (3-8) rows 

 with several valves in a row in the elas- 

 mobranchs (fig. 317, A) and ganoids. 

 This valvular part of the truncus is called 

 the conus arteriosus. In other verte- 

 brates the conus is reduced to a single 

 row of valves. 



Valves also occur in the atrio-ventric 

 ular canal (fig. 316) but here the pocket- 

 sect^S- S'-^'SSZtS;^ like condition is impossible. The folds 

 ventricular valves; a, atnum; ct, extend from the canal into the ventricle 



chorda tendinea; w, rauscula papil- , . , r riTii-^ 



losa; V, ventricle; vl, atrio-ventri- and are prevented from foldmg back mto 

 cular valves. ^^le atrium, under the heavy ventricular 



pressure, by ligaments — chordae tendineafr — which extend from the 

 edges of the valves to the opposite wall of the ventricle, and are kept 

 taut during systole by short papillose muscles (columnae camea) at 

 the base. Other valves, more simple in character, occur around the 

 opening from the sinus into the atrium and, in some vertebrates, 

 where the hepatic veins empty into the sinus. 



In many fishes the conus arteriosus is followed by a strongly 

 muscular region, the bulbus arteriosus (fig 317, B) which has mus- 

 cles Uke those of the heart (p. 130), while the truncus in front of this* 

 has smooth muscles, like the rest of the blood-vessels. Hence conus 

 and bulbus are to be regarded as a part of the heart, while the region 

 in front is a part of ventral aorta to be described below. 



The heart, as described above, is usually spoken of as a branchial 



