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COMPARATIVE ANATOMY 



here is formed what is called tr e atrio-ventricular canal (Fig. 445). It 

 is in front and behind this canal that the walls become thickened and 

 the lumen enlarged. The caudal end, which is also the dorsal in this 

 case, forms the chambers known as the atrium or auricle. The ventral 

 end becomes the ventricle. Caudad in the atrium, there is a constriction 

 forming a second chamber called the sinus venosus and it is into this that 

 the Cuvierian duct and the omphalo-mesenteric vein enter. The ventral 

 parts of the heart-tubes also form a smaller trunk called the truncus 

 arteriosus, while the ventral aorta connects this portion of the heart with 

 the mandibular arteries already mentioned. 



While the heart is really a muscle, or rather many interwound bun- 

 dles of muscles, there are certain parts such as the sinus venosus where 

 the muscle cells themselves 

 are somewhat scanty as 

 compared with other parts 

 of the heart. 



The endocardium devel- 

 ops folds or valves (Fig. 

 445) in certain places so that 

 blood may flow forward but 

 not backward, and this 

 valvular part of the truncus 

 is known as the conus arte- 

 riosus. In the vertebrates 

 this conus is reduced to a 

 single row of valves with 

 the exception of the elasmo- 

 branchs, ganoids, and am- 

 phibia. The valves lie be- 

 tween the auricle and the 

 ventricle and are prevented 

 from being pushed up into 

 the auricle (when the heart contracts and immense pressure is brought 

 to bear upon them) by little ligaments called chordae tendineae, which 

 extend from the edges of the valve to the opposite wall of the ventricle. 

 They are kept taut during systole by capillary muscles called columnae 

 carneae. There is also a valve between the auricle and the sinus in some 

 vertebrates where the hepatic vein enters into the sinus. 



If the conus arteriosus is followed by a strong muscular region this 

 is called the bulbus arteriosus. The bulbus is composed of regular heart 

 muscle, while the truncus is composed of muscle like the rest of the 

 blood vessels. It is for this reason that both conus and bulbus are re- 

 garded as a part of the heart, while anything cephalad to these is con- 

 sidered a part of the ventral aorta. 



Fig. 445. 



A and b, Reduction of the bulbo-ventricular fold of 

 the heart. Ao, aortic bulb; Au, atrium ; B, bulbus cordis; 

 RV, right ventricle; LV, left ventricle; P (in b) pul- 

 monary artery. (After Keith.) 



A, B, C, D, Scheme showing division of bulbus cordis 

 and its thickenings into aorta and pulmonary artery 

 with their valves. The division begins in B, the lateral 

 thickenings dividing respectively into a, e, and c,f. Ro- 

 tation from right to left shown in D. (After Heisler.) 



