NUTRITION. 



scarcely be doubted, therefore, that these are 

 supplied directly from the blood. Dr. Barry 

 and Mr. Addison have spoken with much con- 

 fidence of the metamorphosis of the white cor- 

 puscles of the blood into epithelium-cells; but 

 that this idea is totally inadmissible is proved 

 by the existence of a continuous stratum of 

 basement-membrane, between the capillary net- 

 work and the epidermic or epithelial layer. It 

 is not impossible, however, and perhaps may 

 be considered probable, that the cell- germs 

 contained in this basement-membrane, from 

 which the cells on its external surface appear to 

 take their origin, may be the offspring of the 

 white corpuscles of the blood, which thus sup- 

 plies both the plastic materials and the germs 

 of the constantly-forming new crops of epithe- 

 lial cells. There is no other tissue in the body, 

 after all its organs had attained their full deve- 

 lopement, which can be regarded as taking its 

 origin from the blood in the same degree ; but 

 it may be questioned whether in the formation 

 of new parts, either during the developement of 

 the embryo, or in the reparation of injuries, 

 the office of the blood is not of a similar du- 

 plex character. Thus when plastic lymph is 

 thrown out, between the two surfaces of a 

 wound, the first process, as already mentioned, 

 is its fibrillation ; but at the same time a deve- 

 lopement of cells takes place in it, which cells 

 may possibly undergo a subsequent metamor- 

 phosis into the various forms of tissue which 

 the newly-formed part afterwards contains, 

 precisely as in the first developement of the em- 

 bryonic structure. Such a view, at least, would 

 seem probable in regard to the capillary vessels, 

 which seem to be formed at least as much by 

 the inherent powers of the coagulum, as by the 

 extension of the vessels from the subjacent 

 surface. 



These views are thrown out as hints, rather 

 than as settled ideas. It would be premature, 

 in the present state of our knowledge, to at- 

 tempt to decide questions of such importance 

 without much further examination; and we 

 can only attain a balance of probabilities by 

 interpreting the insufficient results of observa- 

 tion by the aid of the best analogies we can 

 find. The whole subject has made immense 

 progress during the few years which have 

 elapsed since the commencement of the pre- 

 sent work ; but here, as elsewhere, retardations 

 have occurred through hasty generalization and 

 dogmatic assumptions ; and much patient, well- 

 directed, sagacious observation will he needed 

 to unravel the many intricate questions that yet 

 remain to be solved. 



Varying activity of the nutritive processes. 

 Without any change in the character of the 

 nutritive processes which we have been de- 

 scribing, there may be considerable variations 

 in their dtgree of activity ; and this, either as 

 regards the entire organism or individual parts, 

 though most commonly the latter. These va- 

 riations may be so considerable as to constitute 

 disease ; though there are some which take 

 place as part of the regular series of physiolo- 

 gical phenomena. Thus the nutritive processes 

 should have a degree of activity more than suffi- 



751 



to supply the waste of the body during 

 the whole period of infancy, childhood, and 

 adolescence, until, in fact, its full dimensions 

 are attained ; whilst, on the other hand, they 

 are usually less rapid than the disintegrating 

 processes in old age, so that the bulk of the 

 body diminishes. Now as the waste of the 

 body, so far from being more rapid in old age 

 than in childhood, is much less so, it follows 

 that the difference in the activity of the nutri- 

 tive processes in these two states must be very 

 considerable; and this is manifested, not only 

 in the greater demand for food which exists in 

 the child (relatively to the bulk of its body), 

 but also in the greater quickness and facility 

 with which injuries are repaired. Local va- 

 riations may also occur as part of the regular 

 train of vital actions in the adult ; thus we 

 perceive an enormous increase in the amount 

 of tissue contained in the uterus and mammary 

 glands during pregnancy, and a decrease in 

 the bulk of the thymus gland after the first 

 year of infancy. Now in these cases we see 

 that increased nutrition is invariably connected 

 with increased functional activity, and dimi- 

 nished nutrition with diminished functional 

 activity : and this we shall find to be the con- 

 stant rule in regard also to those variations 

 which must be considered as abnormal. 



Increased nutrition, or hypertrophy, is 

 never known to affect the whole body to a de- 

 gree sufficient to constitute disease. It cannot 

 be produced as a consequence of the ingestion 

 of an undue supply of food, for this does not 

 increase the formative activity of the tissues, 

 but merely renders the blood richer in nutritive 

 materials, a part of which the excreting organs 

 are called on to be continually removing, with- 

 out its being rendered subservient to the wants 

 of the body ; whilst another part may be em- 

 ployed in the nutrition of one particular tissue, 

 the adipose, which has a tendency to increase 

 with the superfluity of non-azotized food, pro- 

 vided that the requisite amount of cellular tissue 

 be generated to hold the fatty matter. But 

 examples of hypertrophy of particular tissues 

 or organs are very common. Thus any parti- 

 cular set of muscles which is subjected to fre- 

 quent and energetic use acquires a great in- 

 crease in bulk, as we see in the arms of a black- 

 smith or waterman, the legs of an opera-dancer, 

 &c. The hypertrophy of these muscles is a 

 consequence of their increased functional acti- 

 vity, which being produced by an exertion of 

 the will, and unaccompanied with any inju- 

 rious effects on the system, can scarcely be re- 

 garded as morbid. But there are many in- 

 stances in which the involuntary muscles ac- 

 quire a greatly-increased strength, in conse- 

 quence of an obstruction to their action which 

 results from disease. Thus we see the right 

 ventricle of the heart become hypertrophied 

 (and dilated at the same time) where chronic 

 pulmonary disease produces a difficulty in the 

 propulsion of the blood through the vessels of 

 the lungs ; the muscular fibres of the bladder 

 become enormously hypertrophied, %vhen stric- 

 ture, diseased prostate, or other causes pro- 

 duce a demand for increased expulsive force 



