NUTRITION OF THE TEXTURES xxv 



textures the old substance simply wears away, or is thrown off at tha sur- 

 face, whilst fresh material is added from below. la muscular texture, on 

 the other hand, the process is a chemical or cheinico-vital one ; the func- 

 tional action of muscle is attended with an expenditure of moving force, 

 and a portion of matter is consumed in the production of that force ; that 

 is, it undergoes a chemical change, and being by this alteration rendered 

 uufit 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 mechanicil 

 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 part:?, augments in size and weight : this is " growth." When 

 maturity is attained, the supply of material merely balances the con- 

 sumption ; and, after this, no steady increase takes place, although the 

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

 considerable fluctuation at all periods of life. 



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

 general j we may, however, briefly consider the mode in which the renova- 

 tion 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 sangutnis, which is conveyed by the blood-vessels, and 

 transudes through the coats of their capillary branches ; 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' cuticle and epithelium, the nutritive change is effected by a con- 

 tinuance of the process to which these textures owe their origin. The 

 tissues in question being devoid of vessels, nutrient matter, or blastema, is 

 famished by the vessels of the true skin, or subjacent vascular membrane ; 

 this matter is appropriated 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 conceivable mode in 

 which the renovation of matter might be brought about, namely, by a mole- 

 cular change which renews the substance, particle by particle, without 

 affecting the form or structure ; by a process, in short, which might be 

 termed "molecular renovation." Still, although conclusive evidence is 

 wanting on the point, it seems probable that the crude material of nutrition 

 first undergoes a certain elaboration or preparation through the agency of 

 cells and nuclei disseminated in the tissue ; which may serve as centres of 

 assimilation and increase, as already explained. 



Office of the Vessels. In the instance of cuticle and epithelium, no vessels 

 enter the tissue, but the nutrient fluid which the subjacent vessels afford 

 penetrates a certain way into the growing mass, and the cells continue to 

 assimilate this fluid, and pass through their changes at a distance from, and 

 independently of, the blood-vessels. Whether, in such cases, the whole of the 



