320 
NUTRITION OF NON-VASCULAR TISSUES. 
yet there is considerable variety in the mode in which the 
supply is afforded; some tissues being furnished with blood 
much more copiously and directly than others, in consequence 
of the greater minuteness with which the capillaries are dis¬ 
tributed through their substance. There are several, indeed, 
into which no blood-vessels enter, in their natural state ; but 
which derive their nutriment by absorbing the liquor san¬ 
guinis that is brought into their neighbourhood. This is 
the case, for instance, with the Epidermis and Epithelium 
(§§ 38, 40); the cells of which are developed at the expense 
of the fluid which they absorb, through the basement mem¬ 
brane on which they lie, from the vessels of the skin or 
mucous membrane beneath it. In like manner, even the 
thick layer of Cartilage which covers the ends of most of the 
long bones, is destitute of blood-vessels ; and the small amount 
of nourishment it requires, is obtained by absorption from the 
vessels which surround it (§ 47). This tissue undergoes very 
little change from time to time; and its growth takes place 
chiefly by addition of new matter to its surface; consequently 
there is no necessity for any active circulation through its 
interior; and the transmission of nutritive fluid from one 
cell to another (as in the cellular tissue of Plants) is sufficient 
for its wants. Even in Bone, the blood-vessels are not very 
minutely distributed; for although there is a close network 
of capillary vessels on the membrane lining the Haversian 
canals and the cells of the cancellated structure (§ 49), yet 
none of these pass into the actual substance of the bone. 
The simple Fibrous tissues are, for the most part, sparingly 
supplied with blood-vessels, as they are but little liable to 
decay or injury; though the Areolar tissue serves as the bed 
for the reception of the vessels which are on their passage to 
other tissues. Thus it is by its means that blood-vessels are 
conveyed into the Adipose tissue; for the ultimate elements 
of that tissue, namely, the fat-cells, are surrounded by capil¬ 
lary vessels, not entered by them. The same important pur¬ 
pose is answered by the areolar tissue that lies amongst the 
tubes which form the essential parts of the Nervous and 
Muscular tissues; for these tubes are not perforated by ves¬ 
sels, so that their contents must be nourished by fluid absorbed 
through their walls. 
385. In no instance that we are acquainted with, in the 
