214 



BIOLOGY. [Book ii. 



by those anatomical elements ; and when, speaking of diffusion 

 and osmosis, we have passed in review the principal physical 

 conditions of this nutritive barter. 01. Bernard very justly 

 compares the anatomical elements to animals fixed at the bottom 

 of the sea, and the oxygenised sanguineous wave to the flood which 

 comes incessantly to bathe these animals, bringing to them 

 aliments and respirable air. 



The diverse manifestations of life, and life itself, are intimately 

 dependent on the passage of tM^ nourishing stream. If it is for 

 a moment interrupted in an organ, the functions of that organ 

 are at once suspended, and if the interruption is of some dura- 

 tion, the death of the anatomical elements is inevitable. A liga^ 

 ture applied to the general trunk of the arterial tree, the aorta, 

 in an animal, paralyses the animal instantaneously. Inversely, 

 we can make to revive members amputated and already in a state 

 of cadaveric ijgidity ; we resuscitate an animal's head severed 

 from the trunk by injecting into the arteries, and consequently 

 into the capillaries, either complete blood, or even defibrinised 

 but oxygenised blood. ^ 



But the capillary netwprks, exquisitely fine, are not rigid and 

 inextensible. If they are not contractile, they are assuredly 

 elastic. They can dilate or narrow, according as the sanguineous 

 torrent which traverses them is more or less abundant. Now we 

 have seen that the origins of the capillary vessels themselves, and 

 especially the arterioles whence they spring, are furnished with a 

 contractile tunic. Consequently the calibre of these vessels, and 

 therefore the amount of the sanguineous ciu-rent in the capillaries, 

 must be very variable ; and in truth this is what happens. But 

 the history of the principal conditions of these variations forms 

 one of the most interesting chapters of general physiology, and 

 also one of the most important, forasmuch as it is, so to speak, 

 the mechanism of nutrition which is here concerned. 



Numerous agents' can produce either the contraction or the 



' Brown-Sequai'd, Mecherches sur U EetabZissement de V IrritaMUti Muscu- 

 laire, (CompUm Jiendios et MSmoires de la Societi de Biologic, 1851). 



