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



Fourthly, In the reptiles, as has been demonstrated, the valves are partly 

 tendinous, and partlij muscular ; while in the right ventricle of the bird, they are 

 altogether muscular. Here, then, may be witnessed the first trace of a self- 

 regulating power — actively contracting muscidar fibre, taking the place of non- 

 contractile fibrous tissue. 



In the auriculo- ventricular valves, there is immense variety ; these including 

 most of the forms referred to, and others exhibiting a still higher degree of differ- 

 entiation. They are, for the most part, characterised by the presence of chordae 

 tendinea?, which connect them with the interior of the ventricles, or the structures 

 arising therefrom ; viz. the carnese columnse and musculi papillares. The auriculo- 

 ventricular valves, therefore, differ from the semilunar valves proper. In some 

 instances, only one semilunar flap is present ; and this may be either altogether 

 fibrous, or partly fibrous and partly muscular, or altogether muscular. In a 

 second, there are two flaps or segments, so arranged that their long diameters 

 correspond to the direction of the muscular fibres lining the ventricular cavity ; the 

 segments being continuous with the muscular fibres referred to. In a third, the two 

 segments are attached to the interior of the ventricle by rudimentary chords ten- 

 dinew. In a fourth, two accessory or smaller segments, are added to the two 

 principal ones ; the whole being attached by ivell developed cliordcc tendinece to rudi- 

 mentary muscidi papillares. In a fifth, which is the most perfect form of valve, 

 as it exists in man and in the higher mammalia, the segments are from four to 

 six in number, most exquisitely and symmetrically formed., and attached by minutely 

 graduated chordce tendinece to highly developed cameos columnoe and spiral mus- 

 culi papillares. 



The action of the auriculo-ventricular valves, owing to the want of uniformity 

 in the number, structure, and relations of their segments, is varied. It is, how- 

 ever, on all occasions, carefully adapted to the wants of the circulation, and to 

 the configuration of the ventricles and ventricular cavities ; these cavities, as 

 has been pointed out, adapting or moulding the blood, and causing it to act in 

 given directions. Thus in the fish, where the circulation is slow, and where the 

 ventricle is conical in shape, and composed of fibres interlacing in all directions, 

 the segments, where two are present, are forced towards each other by the uniform 

 expansion of the blood, and by the contraction of the ventricles, in a manner 

 analogous to that by which the segments of the bi-semilunar venous valves are 

 approximated by the retrogressive movements of the slowly advancing venous 

 blood, assisted to a slight extent by the vital contractility of the vessels. 



In the reptile, where the circulation is also languid or slow, the shape of the 

 ventricle, owing to the fibres pursuing a more or less spiral direction, is that 

 of a cone slightly twisted upon itself. As the spiral arrangement extends 

 also to the valves, their action may be aptly compared to that which obtains 

 in the valves of the largest veins, and in the arteries. It is, however, in the 



