16 NUTRITION OF THE TEXTUEES. 



NUTRITION AND REGENERATION OF THE TEXTURES. 



Nutrition. — The tissues and organs of the animal body, when once 

 emploved in the exercise of their functions, are subject to continual 

 loss of material, which is restored by nutrition. This -waste or con- 

 sumption of matter, with which, so to speak, the use of a part is 

 attended, takes place in different modes and degrees m different struc- 

 tures. In the cuticular textures the old substance simply wears away, 

 or is thrown off at the surface, whilst fresh material is added from 

 below. In muscular texture, on the other hand, the process is a 

 chemical or chemico-vital one ; the functional action of muscle is 

 attended with an expenditure of moving force, and a portion of matter 

 derived in part from the muscle itself is consumed in the production of 

 that force; that is„ it undergoes a chemical change, and being by this 

 alteration rendered unfit to serve again is removed by absorption. The 

 amount of matter changed in a given time, or, in other words, the 

 rapidity of the nutritive process, is much greater in those instances 

 where there is a production and expenditure of force, than where the 

 tissue serves merely passive mechanical purposes. Hence, the bones, 

 tendons, and ligaments are much less wasted in exhausting diseases 

 than the muscles, or than the fat, which is consumed in respiration, 

 and generates heat. Up to a certain period, the addition of new matter 

 exceeds the amount of waste, and the whole body, as well as its several 

 parts, augments in size and weight: this is " growth." \yheu maturity 

 is attained, the supply of material merely balances the consumption ; 

 and, after this, no steady increase takes place, although the quantity 

 of some matters in the body, especially the fat, is subject to consider- 

 able fluctuation at all periods of life. 



It would be foreign to our purpose to enter on the subject of 

 nutrition in general; we may, however, briefly consider the mode in 

 which the renovation of substance is conceived to be carried on in the 

 tissues. 



The material of nutrition is immediately derived from the plasma of 

 the blood, or liquor sanguinis, which is conveyed by the blood-vessels, 

 and transudes through the coats of their capillary l>ranches ; and it is 

 in all cases a necessary condition that this matter should be brought 

 within reach of the spot where nutrition goes on, although, as will 

 immediately be explained, it is not essential for this purpose that the 

 vessels should actually pass into the tissue. In certain instances, more- 

 over, the pale corpuscles, which exist in the blood, pass through the 

 coats of the vessels, and may become employed as elements of nutrition 

 and reparation. 



In cuticle and epithelium, the nutritive change is effected by a 

 continuance of the process to which these textures ow^e their origin. 

 The _ tissues in question being devoid of vessels, nutrient matter is 

 furnished by the vessels of the true skin, or subjacent vascular 

 membrane, and is api)ropriated by young cells, derived most probably 

 from pre-existing ones. These new cells enlarge, alter in figure, 

 often also in chemical nature, and, after serving for a time as part of 

 the tissue, are thrown off" at its free surface. 



But it cannot in all cases be so clearly shown that nutrition takes 

 place by a continual formation and decay of the structural elements of 

 the tissue ; and it must not be forgotten, that there is another con- 



