780 DR PETTIGREW ON THE RELATIONS, STRUCTURE, AND FUNCTION, 



valves, the one of which is situated at the beginning, the other at the ter- 

 mination of the bulb. In the bulbus arteriosus of the grey shark {Galeus 

 communis'', we have a slightly different arrangement, the two rows of segments 

 of which the valve is composed being connected to each other by means of 

 tendinous bands, resembling chordae tendinese (Plate XXIX. fig. 48 a). In 

 the bulbus arteriosus of the Lepidosteus (Plate XXIX. fig. 40 b), and that of the 

 basking shark {Selachi maxima, Cuv.), (Plate XXIX. fig. 41 h), the same arrange- 

 ment prevails ; the segments being stronger and less mobile, and the tendinous 

 bands which bind the one segment to the other, more strongly marked than in 

 the grey shark. As the tendinous bands referred to are not in contact with the 

 wall of the bulbus arteriosus, but simply run between the segments, and are in 

 some instances, as in the basking shark, very powerful (Plate XXIX. fig. 41 V), 

 they must be regarded in the light of sustaining or supporting structures ; their 

 function being probably to prevent eversion of the segments. Other examples 

 might be cited, but sufficient have been adduced to show, that the nature, as well 

 as the number and arrangement of the segments, is adapted to the peculiar 

 wants of the structure in which they are situated ; and it ought not to be over- 

 looked, that when a multiplicity of segments are met with in an actively con- 

 tracting organ, the two act together or in unison. 



If we now direct our attention to the auriculo-ventricular valves of the fish 

 and reptile, similar modifications as regards the number of the segments, and 

 the presence or absence of chordae tendinese and analogous structures, pre- 

 sent themselves. Thus in the heart of the serpent {Python tigris), the two 

 crescentic apertures by which the blood enters the posterior or aortic division 

 of the ventricle, are each provided with a single semilunar valve. The same 

 may be said of the aperture of communication, between the left auricle and 

 ventricle of the crocodile (Ci'ocodilus acutus) and of the sturgeon {Accipenser 

 sturio^ Linn.) In the heart of the Indian tortoise {Testudo Indica, Vosmaer), 

 the left auriculo-ventricular orifice is guarded hy a single membranous fold, the 

 right orifice having in addition a slightly yrojecting semilunar ridge, which ex- 

 tends from the right ventricular wall, and may be regarded as the rudiment 

 of the fleshy valve which guards the same aperture in birds (Plate XXIX. fig. 

 45 gli). In the heart of the bulinus, frog-fish, American devil-fish, grey shark, 

 and crocodile, the auriculo-ventricular orifice is guarded by a semilunar valve 

 consisting of two cusps or segments ; while in the sturgeon, sun-fish, and others, 

 it is guarded by four, two larger and two smaller. 



So much for the number of the segments constituting the auriculo-ventricular 

 valves in fishes and reptiles ; but there are other modifications which are not 

 less interesting physiologically. In the bulinus, frog-fish, and crocodile, the 

 segments of the valves are attached to the auriculo-ventricular tendinous ring, 

 and to the sides of the ventricle, and have no chordce tendinece. In the sun-fish 



