CONDITIONS NECESSARY FOR NUTRITION. 385 



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

 border. 



The blood-vessels 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 vascular, the nutritive fluid is carried in streams 

 into the interior ; for the non-vascular, it flows on the sur- 

 face ; but in both alike, the parts themselves imbibe the 

 fluid ; and although the passage through the walls of the 

 blood-vessels 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 dis- 

 tant from all blood-vessels, and can only by imbibition 

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

 the blood-vessels 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 maintains itself, and grows. The instances of the 

 cornea and vitreous humour 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 blood-vessels running into it ; 

 but when it is in thin layers, as in the lachrymal and tur- 

 binated 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 

 itself on every side : a striking instance how, from the 

 same source, many tissues maintain themselves, each exer- 

 cising its peculiar assimilative and self-formative power. 



3. The third condition said to be essential to a healthy 

 nutrition, is a certain influence of the nervous system. 



It has been held that the nervous system cannot be 



