OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. ^01 



Fii'st, An attempt has been made to point out the intimate structm*al rela- 

 tion, existing between the veins, and venous valves ; how the segments of the 

 venous valves are composed principally of white fibrous and yellow elastic tissue 

 arranged in at least three well-marked directions ; and how the segments are so 

 disposed, that their free margins, unless when the blood is actually passing 

 between them, are always more or less in apposition. An attempt has also been 

 made, to demonstrate the nature of the apposition, by the employment of plaster 

 of Paris, injected while in the fluid state into the distal and proximal extremities 

 of the vessels. In the veins, as was pointed out, the closure is to a great extent 

 mechanical; the segments, when two are present, being forced together in the 

 mesial plane of the vessel, by the contraction of its walls, but principally by the 

 weight of the refluent blood. 



Secondly, The structure and relations of the arterial or semilunar valves, 

 particularly in man, have been examined afresh ; a more precise description than 

 that hitherto given of the segments in systematic treatises on anatomy having 

 been essayed. The great vessels have further been shown to bifurcate at their 

 origins, and to be greatly thickened between the segments, which they support 

 and incline towards each other — an arrangement calculated to bring the free 

 margins of the segments more or less closely together, unless when pushed 

 aside by the advancing column of blood during the systole. The sinuses of 

 Valsalva have, in addition, been shown to vary in size ; the one curving towards 

 the other in a spiral direction, and causing the blood to act in spiral waves 

 upon the segments of the semilunar valves, which by this means are twisted and 

 wedged into each other, when the reflux occurs. In the arteries, the action of tlie 

 semilunar valves, as has been explained, is for the most part mechanical, the strong 

 fibrous rings situated at the aortic and pulmonic orifices, tending to counteract 

 the inconvenience, which might be supposed to result from an excess of vital 

 contractility in the vessels, and the ventricles. The arterial semilunar valves, 

 may be said to differ from the bi-semilunar venous ones, in having their segments 

 wedged together by a spiral movement, which in the venous valves, is little more 

 than indicated. 



Thirdly^ The bulbus arteriosus of fishes, has been shown to be a contractile 

 organ, and to contain in its interior a system of valves, the segments of which, are, 

 as a rule, more numerous than in either the veins or arteries. They have, in 

 some instances, tendinous bands, resembling chordte tendinese, running between 

 them ; and are for the most part, arranged in tiers ; so that the blood which is 

 not caught by the one set, falls into, and is supported by the next. The action of 

 these valves, as will readily be inferred, is partly mechanical, and partly vital ; for 

 the contraction of the bulb must be regarded as contributing to the closure. 

 They are, therefore, an advance upon the valves of the veins and arteries, both 

 as regards their number, and the manner of their closure. 



VOL. XXIIL PART IIL 10 E 



