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 Eibrous 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 



