VARYING ACTIVITY OF THE NUTRITIVE PROCESSES. 347 



to it, either in consequence of obstruction in the trunk, or by the par- 

 tial diversion of the stream of blood in another direction ; thus the liver, 

 which is much more developed in the foetus, relatively to the rest of the 

 body, than it is in the adult, undergoes a partial atrophy immediately 

 after birth, in consequence of the change in the whole course of the cir- 

 culation, which takes place as soon as the lungs are expanded. 



620. But partial atrophy may also take place from causes inherent 

 in a particular organ. Thus we occasionally meet with limbs, which 

 are " blighted," never attaining their due size relatively to the remainder 

 of the body, yet not exhibiting any defect of organization. Here there 

 would seem to be, from some unknown cause, a deficient power of 

 growth ; analogous to that which, when acting on the body in general, 

 confines it within dwarfish dimensions. One of the most frequent causes 

 of partial atrophy, however, is want of functional activity in the organ ; 

 and this is particularly the case in regard to the Muscular and Nervous 

 systems. Thus, as already remarked ( 348), any set of Muscles that 

 is long disused, becomes partially atrophied ; which is probably due to 

 the feebleness and languor of the circulation, consequent upon the 

 absence of the demand for arterial blood. As soon as the parts are 

 called into use again, their nutrition improves. So, also, in regard to 

 the Nerves ; the nutrition of both the fibrous and vesicular structures 

 appears to be entirely dependent upon the activity of their function ; 

 and as the former are inert without the latter, we may say that the due 

 nutrition of the nervous system entirely depends upon the functional 

 activity of the vesicular matter. Of this we have a well-marked illus- 

 tration in the fact, that when the cornea has been rendered so opaque 

 by disease or accident, as to prevent the penetration of any light to the 

 interior of the eye, the retina and the optic nerve lose after a time their 

 characteristic structure ; so that scarcely a trace of the peculiar globules 

 of the former, or of the nerve-tubes of the latter can be found in them. 



621. In the healthy condition of the organism, however, the nutri- 

 tion, in every part of the body goes on in a degree sufficient to keep it 

 constantly ready for the performance of its appropriate function ; a 

 regular supply of the requisite materials being furnished in the aliment, 

 and being prepared by the assimilating processes ; and the products of 

 the waste or decay of the tissues, together with such alimentary mate- 

 rials as may be superfluous, being carried off by the excreting opera- 

 tions. When the nutrition and the waste are eqfual, the weight of the 

 body remains the same ; and this is commonly the case in adult age. 

 But during the earlier periods of life, the powers of growth are greater ; 

 the demand for food is very large in proportion to the bulk of the body ; 

 and though the waste is rapid, and the excreting processes very active 

 (as evinced by the large amount of urea and of carbonic acid set free), 

 the growth predominates over the decay, and the development of the 

 whole structure proceeds at a gradually-decreasing rate, until the full 

 stature and bulk are attained. The energy of the nutritive process is 

 particularly manifested, in the rapidity and completeness with which 

 severe injuries, occasioned by disease or accident, are repaired. In 

 advanced life, on the contrary, although the waste is comparatively 

 small, the renewing processes are enfeebled in a still greater degree ; 



