STRUCTURE AND DEVELOPMENT OF CAPILLARY BLOODVESSELS. 299 



branous parietes. These tubes commonly form a minutely-anastomosing network; 

 into which the blood is brought by the ramifications of the arteries on one side, 

 and from which it is returned by the radicles of the veins on the other. The 

 walls of the tubes are composed of a delicate membrane, in which an appearance 

 of transverse striation (as if produced by minute annular fibres) can sometimes 

 be discerned. The diameter of the Capillaries varies in different animals, in 

 accordance with that of their blood-corpuscles; thus the Capillaries of the Frog 

 are, of course, much larger than those of Man. The diameter of the latter ap- 

 pears, from the measurements of Weber, Miiller, and others, to vary from about 

 the l-3700th to the l-2500th of an inch; but as they can only be examined after 

 death, it is probable that these statements are not altogether exact, particularly 

 as tubes of the smallest of the above sizes would not admit ordinary blood-cor- 

 puscles. The dimensions of the individual vessels, indeed, are by no means con- 

 stant ; as may be seen by watching the Circulation in any transparent part, for 

 some little time. Putting aside the general changes in diameter, which result 

 from circumstances affecting all the capillaries of a part, it may be observed that 

 a single capillary will sometimes enlarge or contract by itself, without any ob- 

 vious cause. Thus, the stream of blood will sometimes be seen to run into 

 passages, which were not before perceived; and it has hence been supposed that 

 they were new excavations, formed by the retreating or removal of the solid tis- 

 sue through which its passes. But a more attentive examination shows, that 

 such passages are real capillaries, which did not, at the time of the first obser- 

 vation, admit the stream of blood-corpuscles, in consequence of the contraction 

 of their caliber, or of some other local impediment; and that they are brought 

 into view by the simple increase in their diameter. The compression of one of 

 the small arteries will generally occasion an oscillation of the corpuscles of blood 

 in the smallest capillaries, which will be followed by the disappearance of some 

 of them; but when the obstruction is removed, the blood soon regains its pre- 

 vious velocity and force, and flows into exactly the same passages as before. 



293. The opinion was long entertained, that there are vessels adapted to sup- 

 ply the white or colorless tissues; carrying from the arteries the " liquor san- 

 guinis," and leaving the corpuscles behind, through inability to receive them. 

 But such a supposition is altogether groundless. Some of the white tissues, as 

 Cartilage, are altogether destitute of vessels; and in others, the supply of blood 

 is so scanty, as not to communicate to them any decided hue. It is evident 

 from what has been already stated, that the idea that Nutrition can only be car- 

 ried on by means of Capillary vessels, is entirely gratuitous. There is no essen- 

 tial difference, in fact, between the nutrition of the non-vascular tissues, and that 

 of the islets in the midst of the network of capillary vessels which traverses the 

 most vascular. In both cases, the nutrient materials conveyed by the blood are 

 absorbed by the cells or other elementary parts of the tissue immediately ad- 

 joining the vessels, and are imparted by them to others which are further removed; 

 and the only difference lies in the amount of the portion of tissue which has to 

 be thus traversed. There is great variety in this respect, as we have seen, 

 among the vascular tissues ; and we are only required to extend our ideas, from 

 the largest of the islets which we find in these, to the still more isolated struc- 

 tures of which the non-vascular tissues are composed. The distribution of Ca- 

 pillaries through the vascular tissues, and the character of the reticulation which 

 they form, vary so greatly in the different parts of the fabric, that it is possible 

 to state with tolerable certainty the nature of the part from which any speci- 

 men has been detached whether a portion of skin, mucous membrane, serous 

 membrane, muscle, nerve, fat, areolar tissue, gland, &c. But the arrangement 

 of vessels peculiar to each, evidently has reference only to the convenience of 

 the distribution of blood among the elementary parts of the tissue, and varies 

 with their form. It is not possible to imagine that it has any other relation than 



