Organs of Respiration and Circulation. 



5S3 



a multiplicity of facts, were it necessary, to prove the correctness 

 of Hunter's views, but content myself by stating that I am of 

 the number of his disciples.* 



As the arteries everywhere terminate in capillaries, so do we 

 find that the veins arise from them. A vein (see b, fig. 5) differs 

 materially from an artery, first in the thickness of its coats, and 

 secondly in having its internal lining thrown into folds here and 

 there, forming thereby its valves, marked c, fig. 5. These valves 

 perform the office of such structures in general, namely, that of 

 allowing a fluid to pass but in one direction ; and as their free 

 edges are directed towards the heart, it follows that they prevent 

 any retrograde motion in the blood by rising and closing the 

 canal. This arrangement is rendered the more necessary in con- 

 sequence of veins not exerting any power per se in the return of 

 the blood, this being chiefly effected by their being pressed upon 

 by the various muscular movements of the body. Veins are also 

 non-pulsatory, and the stream of the blood through them is con- 

 tinuous and even. They are far more numerous than arteries, and 

 are divisible into a superficial and deep-seated set, which freely 

 communicate by anastomosing branches. They likewise increase 

 in size, but diminish in number as they approach the heart, near 

 to which those of the system ultimately terminate in the two cavoe. 

 The blood which they carry is dark in colour, unlike that of the 

 arteries, in which vessels it is of a scarlet hue. This change in 

 the colour of the fluid is produced in the capillaries ; the cause 

 and the consequence of which we shall now consider. Fig. 6 



Fig. 6. 



Capillaries -ef fat. a. The terminal artery. b. The primitive vein. 

 (From Todd and Bowman's ' Physiological Anatomy.') 



* Since this lecture was delivered, an opportunity has been afforded the author, hy 

 the death of the rhinoceros in the gardens of the Zoological Society, of confirming these 

 opinions. He examined a portion of the carotid artery, and found its muscular coat 

 extensively developed. The fibres were arranged more or less in a circular order. 



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