NUTRITION. 



77 



venous blood ramifying through vessels disposed like arteries, and 

 having to overcome the resistance of an additional capillary system 

 in the liver, before reaching the heai't. 



The erectile tissues appear essentially to consist of a plexus of 

 varicose veins, enclosed in a fibrous envelope, which plexus, ac- 

 cording to Gerber, is traversed by numerous contractile fibres, to 

 the contraction of which is probably to be attributed that obstruction 

 to the return of blood by the veins, which is the occasion of their 

 turgescence. In the penis, according to Muller, there are two sets 

 of arteries, one destined for the nutrition of the organ, the other, by 

 communicating with venous cells, for its erection. In all the erectile 

 tissues, erection may ensue either from local irritation, or as a result 

 of certain emotional conditions of the mind, the influence of which is 

 probably transmitted through the sympathetic nerve. 



NUTRITION. 



According to the definition of Adelon, nutrition is the action by 

 which every part of the body, on the one hand, appropriates or assi- 

 milates to itself a portion of the blood distributed to it, and, on the 

 other hand, yields to the absorbing vessels a portion of the materials 

 that previously composed it. The process of nutrition is not an ob- 

 ject of microscopic observation ; the precise mode, therefore, in 

 which it is accomplished is not accurately known. The source of 

 all nutrition and of all growth is the bloody from which materials are 

 shed or separated, to be employed in the renovation and repara- 

 tion of the tissues. This is obviously only to be accomplished by 

 the parenchyma selecting from the capillaries and intermediate ves- 

 sels those ingredients that can become inservient to this process. 

 The structure composing every separate portion of the body has, 

 what may be called, an elective affinity for some particular consti- 

 tuents of the blood ; causing it to abstract from that fluid, and to 

 convert into its own substance, certain of its elements. This selecting 

 power, possessed by the component cells of every tissue, is exercised 

 not only upon the materials required for their development, but even 

 upon substances abnormally present in the blood ; thus, arsenic will 

 produce irritation of the mucous membranes of the body ; and the 

 continued introduction of lead into the circulating system modifies the 

 nutrition of the extensor muscles of the fore-arm, producing the phe- 

 nomena of lead palsy ; the existence of this modification is proved 

 by chemical analysis, which reveals the existence of lead in the 

 palsied muscles. Substances thus introduced into the blood affect 

 the symmetrical portions of the body ; thus the extensor muscles in 

 both arms are paralysed ; and the cutaneous eruptions produced by 

 the internal exhibition of certain remedies, are found to be almost 



precisely symmetrical j the presence of the medicine in the blood 



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