OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBEATA, 795 



and elasticity of the vessels. In the bulbus arteriosus of fishes, where the area 

 of activity of the valves is not thus circumscribed, and where they are exposed 

 to the influence of muscular contraction, the segments are not only inc7'eased 

 in mimbe?', but cliordce tendinece, in the shape of tendinous bands, begin to make 

 their a'piyearance. In the auriculo-ventricular valves of fishes and reptiles, 

 chordcB tendinece in various stages of development occur, these being attached to 

 the interior of the ventricle to more or less fidly developed muscidi papillares ; 

 the musculi papillares, which occur only in the reptilia, being in no instance 

 so well marked as in the ventricles of the aves and in the mammalia. As we 

 thus rise in the scale of being, and the requirements of the circulation become 

 greater, it will be observed that the relation of the segments to actively contracting 

 structures, becomes more and more defined. In the ventricle of the fish, as I 

 pointed out, the fibres proceed in wavy lines from base to apex, and from apex 

 to base, from without inwards and circularly ; so that the organ contracts and 

 dilates very much as one mould shut and open the hand. In the reptilia, the 

 external and internal fibres pursue a slightly spiral direction— ^/^e ventricles 

 rotating more or less when in action. In the cold-blooded animals, moreover, as 

 every one is aware, the circulation is languid or slow, so that an arrangement 

 of valves similar in some respects, though more complex than that which exists 

 in the veins and venous sinuses and in the arteries, amply suffices. In the hearts, 

 however, of the warm-blooded animals, where the ventricles are composed entirely 

 of spiral fibres, and where the circulation, on account of the sudden twisting 

 and untwisting of the fibres is very rapid., a system of valves, which will act with 

 greater rapidity and precision, is absolutely necessary. But functional precision 

 implies structural excellence ; and hence that exquisite arrangement of parts in 

 the auriculo-ventricular valves of mammals, whereby every portion of every 

 segment (by reason of the ever varying length and strength of the chordae 

 tendineee) bears a graduated relation to the musculi papillares and carnese 

 columnse. Although the partial closing of the valves during the diastole may be, 

 and is occasioned by the uniform expansion of the blood owing to the force 

 exercised upon it by the contraction of the auricles, still it must be evident to all 

 who reflect, that this cause is not of itself adequate to the complete closure, and 

 for a very obvious reason. The blood, which is the expanding force, derives its 

 power solely from the contraction of the auricles, and enters the ventricular 

 cavities by the auriculo-ventricular orifices. Once in the ventricles, however, 

 the blood has no inherent expansive power, by which it can of its own accord 

 entirely shut off or close, the apertures by which it entered. This act, as I shall 

 show presently, requires for its consummation, the force exercised by the con- 

 traction of the ventricles at the commencement of the systole. Admitting, how- 

 ever, that the expansion of the blood was adequate to the closure of the auriculo- 

 ventricular valves at one period,— say at the end of the ventricular diastole, 



