CONDITIONS NECESSARY FOR NUTRITION. 307 



the normal constitution of the blood, should be immediately 

 reflected, so to speak, as a change in the nutrition of the 

 solid tissues and organs which it is destined to nourish. 



2. The necessity of an adequate supply of appropriate blood 

 in or near the part to be nourished, in order that its nutrition 

 may be perfect, is shown in the frequent examples of atrophy 

 of parts to which too little blood is sent, of mortification or ar- 

 rested nutrition when the supply of blood is entirely cut off, 

 and of defective nutrition when the blood is stagnant in a part. 

 That the nutrition of a part may be perfect, it is also neces- 

 sary that the blood should be brought sufficiently near to it 

 for the elements of the tissue to imbibe, through the walls of 

 the bloodvessels, the nutritive materials which they require. 

 The bloodvessels themselves take no share in the process of 

 nutrition, except as carriers of the nutritive matter. There- 

 fore, provided they come so near that this nutritive matter 

 may pass by imbibition into the part to be nourished, it is 

 comparatively immaterial whether they ramify within the 

 substance of the tissue, or are distributed only on its surface 

 or border. 



The bloodvessels serve alike for the nutrition of the vascular 

 and the non-vascular parts, the difference between which, in 

 regard to nutrition, is less than it may seem. For the vascu- 

 lar, the nutritive fluid is carried in streams into the interior ; 

 for the non-vascular, it flows on the surface ; but in both alike, 

 the parts themselves imbibe the fluid ; and although the pas- 

 sage through the walls of the bloodvessels may effect some 

 change in the materials, yet all the process of formation is, 

 in both alike, outside the vessels. Thus, in muscular tissue, 

 the fibrils in the very centre of the fibre nourish themselves : 

 yet these are distant from all bloodvessels, and can only by 

 imbibition receive their nutriment. So, in bones, the spaces 

 between the bloodvessels are wider than in muscle ; yet the 

 parts in the meshes nourish themselves, imbibing materials 

 from the nearest source. The non-vascular epidermis, though 

 no vessels pass into its substance, yet imbibes nutritive matter 

 from the vessels of the immediately subjacent cutis, and main- 

 tains itself, and grows. The instances of the cornea and vitre- 

 ous humor are stronger, yet similar ; and sometimes even the 

 same tissue is in one case vascular, in the other not, as the 

 osseous tissue, which, when it is in masses or thick layers, has 

 bloodvessels running into it ; but when it is in thin layers, as 

 in the lachrymal and turbinated bones, has not. . These bones 

 subsist on the blood flowing in the minute vessels of the mucous 

 membrane, from which the epithelium derives nutriment on 

 one side, the bone on the other, and the tissue of the membrane 



