VI 



HEART 



389 



Amniota, namely that the conus arteriosus has gradually lost its 

 power of rhythmic contraction while pari passu its myoeardiac 

 coating of striped muscle has degenerated and its primitive histo- 

 logical characteristics have been replaced by others resembling more 

 closely those of the ventral aorta. 



During ontogeny it would appear from H oyer's work on Salmo 

 (1900) that the conus in the embryo possesses the characteristic 

 features — a layer of striated muscle in its wall, and longitudinal 

 ridges (two in number) projecting info its lumen— and differs from 

 that of an Elasmobranch merely in 

 the fact that these features do not 

 extend throughout the whole of the 

 distance between the ventricle and 

 the anterior limit of the pericardiac 

 space, but only through about the 

 posterior half of that distance. In 

 the adult the two ridges are repre- 

 sented by the two pocket-valves. 



In Urodele amphibians (Sala- 

 mandra — Hochstetter, 1906) the 

 heart during the period when it is 

 in the form of a tube with an S-like 

 curvature ;s conspicuously different 

 in appearance from that of the 

 Vertebrates already described, owing 

 to the fact that the two curves of 

 the S lie in different planes from 

 those which they occupy elsewhere. 

 The morphologically posterior or tail- 

 ward curve lies here nearly in the 

 horizontal plane while the anterior 

 curve lies in a nearly vertical plane 

 — the limb of the curve which will 

 become conus lying dorsal to the 

 ventricular portion, so that it is 

 hidden in a view of the heart from 

 the ventral side (Fig. 184, A). Special interest is lent to this 

 curvature of the atrioventricular portion of the cardiac tube by the 

 fact that it reproduces accurately the type of curvature which we 

 inferred as being present in this portion of the vertebrate heart in 

 our general discussion of its morphology (compare Fig. 184, A, with 

 lower portion of Fig. 177). 



As development proceeds the left-hand end of the ventricular 

 portion, i.e. its actually headward end, swings ventrally and tailwards 

 so that the long axis of this portion of the heart comes to be per- 

 pendicular to the sagittal plane of the body (Fig. 184, B). As the 

 ventricular part of the heart shifts backwards the conus becomes 

 visible in a view from the ventral side. The backward shifting 



Fig. 184.— Views of developing heart of 

 Salamandra as seen from the ventral 

 side. (After Hochstetter, 1906.) 



at, atrium; c, conus arteriosus; d.C, duct 

 of Cuvier; p.v.c, posterior vena cava; s.v, 

 sinus venosus ; V, ventricle. 



