IX 



SKELETON, HEART 



321 



The heart is rendered efficient as a pump by being provided with 

 valves which ensure the blood passing always in the same direction and 

 prevent regurgitation^ i.e. the passing backwards of the blood in the 

 wrong direction. Two of these, in the forrh of flaps opening downwards, 

 are present in the atrioventricular opening. Six others — semilunar or 

 pocket-valves (Fig. 134, p.v) — are situated in the conus, arranged in 

 two circles of three valves each. They may also be described as being 

 arranged in three longitudinal rows of two valves each, and this is a 

 better mode of description for it draws attention to an important morpho- 

 logical fact. The study of development shows that in the embryo each 

 row of valves is represented by a longitudinal ridge. After the ventricle 



ua. 



P^~' 



P-9- 



h.w. 



Fio. 134. 



Diagrammatic sagittal (i.e. longitudinal, vertical, median) section through heart of a Dogfish 

 [Acanthias). a. Atrium; b.w, body-wall; c, conus arteriosus; d.C, duct of Cuvler ; o, floor of 

 oesophagus ; p, pericardiac cavity ; p.c, peritoneal cavity ; p.g, pectoral girdle ; p.v, pocket-valves ; 

 ppc, pericardio-peritoneal canal ; s.v, sinus venosus ; v,a, ventral aorta. 



has contracted and driven the blood forwards the wall of the conus 

 contracts and jams the three ridges together so as completely to obliterate 

 the cavity and effectually prevent any sucking back of the blood into 

 the ventricle as it expands. As development goes on the greater part 

 of each ridge flattens out and disappears but two portions persist in a 

 modified form as the two pocket- valves. Hence the fact that these are 

 in line with one another. Each pocket-valve is shaped like a watch- 

 pocket opening in a headward direction. As the blood passes forwards 

 the pocket-valves flatten against the wall of the conus and interpose no 

 resistance to the blood-stream, but the moment any sucking back of the 

 blood commences the pockets open out and their edges coming together 

 block the cavity. 



Y 



