742 



NUTRITION. 



lion is required, for the maintenance of 

 their normal texture and properties; but its 

 amount will vary, according to the demand 

 created by previous activity, and the conse- 

 quent decay. 



The materials required for the nutrition of 

 the tissues of the animal body seem to be sup- 

 plied, for the most part at least, in forms pos- 

 sessing a similar chemical composition, by the 

 vegetable kingdom. It will be presently shown 

 that albumeii may be regarded as the pabni/im, 

 at the expense of which all the organised tex- 

 tures (properly so called) of the animal fabric 

 maybe constructed. The really-organised part 

 of this fabric, indeed, appears seldom to de- 

 part widely from the protein type of composi- 

 tion. Thus in fat, the non-azotized matter is 

 contained within cells, whose walls are com- 

 posed of a protein principle ; and in the ner- 

 vous tissue, it is probable that the walls of the 

 cells and tubes are composed of an albuminous 

 compound, though their interior is occupied by 

 a substance of a character much more nearly 

 approaching that of fat. Even with respect to 

 the gelatinous tissues, as they are termed, 

 there is much doubt to what extent they con- 

 tain gelatin in their normal state; for where 

 this can only be extracted from them by long 

 boiling, it is not improbable that an actual con- 

 version takes place ; since we know that pure 

 fibrin may be converted, by long boiling, 

 (which occasions the liberation of ammonia,) 

 into a compound resembling gelatin in many 

 respects. And in those which are most purely 

 gelatinous, it is doubtful how far the gelatin is 

 itself organized. The writer has lately ex- 

 amined the sound of a cod with great care, both 

 before and after the action of hot water upon 

 it, and is satisfied that the gelatinous portion 

 of it exhibits nothing that can be properly 

 called organization the only distinct appear- 

 ances of fibres, cells, &c., being presented by 

 portions which were left undissolved by the hot 

 water, and which were, therefore, to be regarded 

 as more allied to albumen than to gelatin in 

 their composition. Similar remarks may be 

 made in regard to the horny substance depo- 

 sited in certain tissues ; and it may probably 

 be stated as a general theorem, that whilst in 

 the plant, the materials which it derives from 

 the elements around are combined and elabo- 

 rated into non-azotized compounds for the pro- 

 duction of organized tissue, and into azotized 

 products for deposition in its cavities, these 

 last alone form the materials of the animal or- 

 ganism, any non-azotized substances contained 

 in it being inorganic in their condition. 



In considering the various stages of the nu- 

 tritive process in animals, we shall do well to 

 bear constantly in mind the leading facts in re- 

 gard to the same process in the simplest cellu- 

 lar plant : for we shall find that the elementary 

 parts of the most complex animal organism go 

 through a series of changes essentially the same ; 

 so that the type of the function is everywhere 

 uniform, notwithstanding the vast apparent dif- 

 ferences in the mode in which it is performed. 

 The cell of the red snow or yeast plant, for 

 instance, is developed from an almost imper- 



ceptible germ, by its own power of attracting 

 to itself certain nutritive materials in its neigh- 

 bourhood, which it combines into the new 

 forms required both for its own growth and in- 

 crease, for the elaboration of certain peculiar 

 matters contained in its cavity, and for the pro- 

 duction of the germs of new cells; and these, 

 being liberated in time by the death of their 

 parent, go through, in their turn, the same series 

 of changes. We shall now trace these changes 

 in the highest and most complex form in which 

 they are presented to us; that is, as they oc- 

 cur in man, or any vertebrated animal. 



Elaboration of organizable materials. The 

 alimentary substances taken in by the absorbent 

 vessels require to undergo very important 

 changes within the body, before they can be 

 applied to the nutrition of its structure. The 

 chief constituents of the chyle, as at first ab- 

 sorbed, are albumen and fat; the former is 

 destined to be converted into the material of 

 the solid tissues ; the latter is chiefly designed 

 for the maintenance of the animal temperature, 

 by the combination it is made to undergo with 

 the oxygen introduced through the lungs. It 

 is questionable, as already explained, whether 

 fatty matter, or any other non-azotized com- 

 pound, can ever be applied to the nutrition of 

 the animal body. Even if it should be ever 

 proved to be subservient to the re-construction 

 of the azotized tissues, there can be no doubt 

 that it must have been first converted into an 

 albuminous compound that is, into some mo- 

 dification of protein; and as the evidence that 

 such a transformation ever takes place is far 

 from bein'4 satisfactory, we have as yet no data 

 for examining the mode in which it is effected. 

 We shall, therefore, consider albumen as the 

 starting-point of the animal tissues, and shall 

 endeavour to trace, so far as the present state of 

 our knowledge admits, the processes by which 

 this is converted into the organized fabric. 



In this assumption we seem justified by two 

 very obvious considerations. First, in the egg 

 of a bird, (or any other oviparous animal,) we 

 find that, putting aside the fatty matter of the 

 yolk, albumen is the sole organic compound, at 

 the expense of which all its tissues are to be 

 formed ; so that, by the wonderful processes of 

 chemical and vital transformation, which take 

 place during the period of incubation, the al- 

 bumen which it contained at first is meta- 

 morphosed into bone, cartilage, nerve, muscle, 

 tendon, ligament, membrane, areolar tissue, 

 gelatinous matter, horny substance, feathers, 

 &c.,&c. Secondly, a similar metamorphosis 

 appears to be continually taking place in the 

 body of the adult animal; for every protein 

 compound employed as food appears to be re- 

 duced to the form of albumen in the digestive 

 process ; so that this becomes the essential con- 

 stituent of whatever fluid is absorbed for the 

 nutrition of the tissues. It is true that gelatin, 

 taken in as food, may be absorbed and carried 

 into the current of the circulation ; but there is 

 no doubt that it is altogether incapable of being 

 applied to the re-construction of any but the 

 gelatinous tissues ; and, as already stated, it 

 seems questionable whether, even in these, it 



