344 



OF THE CIRCULATION OF THE BLOOD. 



FIG. 151. 



those of the retina and brain, are composed of the intimately-adhering cells 

 alone, without any basement-membrane, this last only becoming perceptible 

 in those of larger diameter. It is possible that minute interspaces may exist 

 between adjoining cells, through which, when the pressure of the blood is 

 increased, the white and even the less-yielding red corpuscles may escape. 1 

 Non-medullated nerve-fibres have been traced by Beale to the capillaries, 

 and by the aid of the gold method of staining, Klein has followed still finer 

 fibres into the substance of the capillary wall. 2 The diameter of the capil- 

 laries varies in different animals, in accordance with that of their blood-cor- 

 puscles ; thus the capillaries of the frog are, of course, much larger than 

 those of Man. The ordinary diameter of the latter appears, from the meas- 

 urements of Weber, Miiller, and others, to vary from about the g^gth to the 

 Ti-g-^fith of an inch ; the extremes, however, are stated by Kolliker at as little 

 as -g-g'dijth and as much as y?rroth f an inch. As the diameter of the human 

 capillaries, however, can only be examined after death, it is probable that 

 these statements may not be altogether correct, particularly as tubes of the 



smallest of the above sizes would 

 not admit ordinary blood-corpus- 

 cles. The dimensions of the indi- 

 vidual vessels, indeed, are by no 

 means constant; as may be seen 

 by watching the circulation in any 

 transparent part, for some little 

 time. Putting aside those general 

 changes in diameter which result 

 from circumstances affecting all the 

 capillaries of a part, it may be ob- 

 served that a single capillary will 

 sometimes enlarge or diminish by 

 itself, without any obvious cause. 

 Thus the stream of blood will some- 

 times 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 re- 

 moval of the solid tissue through 

 which it passes. But a more at- 

 tentive examination shows, that 



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

 observation admit the stream of blood-corpuscles, in consequence of their 

 small calibre, 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 disappear- 

 ance of some of them ; but when the obstruction is removed, the blood soon 



A. Fine capillaries from the mesentery, u. Capil- 

 laries of larger size auil with thicker walls, from the 

 pecten of the eye of a bird. 



chow's Arcliiv, 186P>, Bd. xxxv, p. 171 ; Auerbaeh, Kreslau. Zeituni;-, 1815"); Achy, 

 Medicin. Ontralblatt, 18f>f>, No. 14; Colinheitn, Virclmw's Arcliiv, Bd. xv, p. 52, 

 and Uaiivicr and ('ornil, Brown-Sequard, Arcliiv. dc Physiol., t. i, 18(18, p. r>">l. 



1 See Arnold, Virchow's Archiv, Band Iviii, 1873, p. 220, and Frey, Barker's 

 Transl , 1875, p. :-J(>4, Fig. 358. 



' Klein, Distribution of Non-medullated Ncrvo-tibres, Quart. Journ. of Mic. Sci., 

 No. Ixvi, 1872, p. 12"). An appendix to thi.s paper contains a list, of thirty-eight 

 memoirs on this subject. Beale, Tlio Nerves of Capillary Vessels, etc., Monthly 

 JVlicroscop. Journ., 1872, vol. viii, p. 57. 



