PHYSIOLOGY. 



bit and conformation. For comparative 

 anatomy shows us instances of the same 

 fluid secreted by organs of very different 

 obvious structure. 



How, or why, certain organs secrete 

 certain liquors, is the most important and 

 essential question in this subject ; but one 

 to which our ignorance will not enable 

 us to reply in a satisfactory way. Proba- 

 bly the chief and proximate cause con- 

 sists in difference of structure, and per- 

 haps in the arrangement of the minute 

 vessels, which are the organs of secretion. 

 The peculiar powers of each part, its 

 share of irritability, and contractility, 

 must also have an important influence. 

 The mechanical explanation of the phe- 

 nomenon, by the straining of the fluids 

 through different sized pores, cannot be 

 admitted for a moment. We have one 

 fluid, the blood, sent into different or- 

 gans ; each of which separates from it a 

 different produce of matter, differing in 

 many instances from any contained be- 

 fore in the blood. Here then must be a 

 decomposition and a recombination of 

 elements produced by the living action of 

 the gland. 



Nutrition may be considered as the 

 completion of the assimilating functions ; 

 to which the processes hitherto describ- 

 ed, under the heads of Digestion, Ab- 

 sorption, Circulation, Respiration, and 

 Secretion, are only preliminary and pre- 

 paratory. The food, changed in the man- 

 ner we have already described, anamalised 

 and rendered similar to the being which 

 it is designed to nourish, applies itself to 

 those organs whose losses it is to supply, 

 and this identification of nutritive matter 

 to our organs constitutes nutrition. The 

 component parts of the living body are 

 incessantly carried off by various causes. 

 Thus the machine is continually destroy- 

 ed, and at distant periods of life does not 

 contain any of its original elements. Mad- 

 der, mixed with the food, dyes the bones 

 of a red colour, which disappears when 

 the use of the root is suspended. These 

 phenomena can only be explained by ad- 

 mitting an entire removal and renewal of 

 the bony particles. Now if the most com- 

 pact and solid parts be in a continual mo- 

 tion of decomposition and recomposition, 

 this motion must be more rapid where 

 the constituent principles are in the 

 smallest degree of cohesion, as in fluids. 

 Physiologists have endeavoured to deter- 

 mine the period of the entire renovation 

 of the body, and have considered that an 

 interval of seven years is necessary for 

 the original particles to be totally ob- 



literated, and their place supplied by 

 others. 



When the nutritive matter has been 

 duly assimilated, the parts which it sup- 

 plies retain it, and incorporate it with 

 their own substance. This nutritive ap- 

 propriation is variously effected in differ- 

 ent structures ; since each part converts 

 to its own use, by a true secretion, that 

 which is found analagous to its nature, 

 and rejects the heterogeneous particles. 



The mechanism of nutrition would be 

 explained, if we could understand how 

 each function divests the aliments on 

 which we exist of their characters, to in- 

 vest them with the properties of our or- 

 gans ; how each individual part co-ope- 

 rates in changing their nutritious princi- 

 ple into our own peculiar structure. Ve- 

 getables, which form the sole nourish- 

 ment of man in many instances, and a ve- 

 ry great share of it in all cases, consist 

 chiefly of carbon, hydrogen, and oxygen, 

 with sometimes a small quantity of azote 

 and salts. In the organs of the man fed oir 

 these vegetables, azote predominates, 

 and many new products are discovered, 

 not distinguishable in the aliment, and, 

 therefore, formed in the act of nutrition. 

 Every living body, without exception* 

 possesses this faculty of forming and de- 

 composing substances, and of giving rise 

 to new products. 



The marine plant, whose ashes form so- 

 da, if sown in a box filled with earth that 

 does not contain a particle of that alkali, 

 and moistened with distilled water, fur- 

 nishes it in as great a quantity as if the 

 plant had been growing on the borders 

 of the sea, and always supplied with salt 

 water. Living bodies then are elaborato- 

 ries, in which such combinations and de- 

 compositions occur, as art cannot imitate ; 

 bodies that to us appear simple, as soda 

 and silex, seem to form themselves of 

 other parts ; while some, whose compo- 

 sition we cannot determine, as certain 

 metals, suffer inevitable decompositions ; 

 from which we may fairly conclude, that 

 the powers of nature in the composition 

 and decomposition of bodies far surpass 

 the science of chemists. 



SEJfSATIOSS. 



Vision. The mode in which the rays of 

 light are affected, in passing through the 

 various parts of the eye, is explained un- 

 der the article OPTICS. See also VISIOK. 

 We have only to add a few remarks on 

 the physiology of the eye. 



The quantity of light that can enter the 



Z 2 



