OF THE CIRCULATION IN GENERAL. 301 



distinct parietes, and that these are composed of a fibrous structure analogous 

 to the muscular. Their mode of origin, again, refutes such a supposition; 

 for there can be no doubt that they are produced, in any newly forming 

 tissue, not by the retirement of its cells, one from the other, so as to leave 

 passages between them, but by the formation of communications among cer- 

 tain cells, whose cavities become connected with each 

 other, so as to constitute a plexus of tubes, of which Fig. 90. 



the original cell-walls become the parietes. The pro- 

 cess may be most clearly traced in Plants, in which 

 (among the Phanerogarnia at least) a distinct system 

 of capillary vessels exists ; but it may also be seen in 

 the germinal membrane of the Ovum, in which the 

 capillaries distinctly originate before the trunks ; and 

 this view of it is confirmed by the fact, that the nuclei 

 or cytoblasts of the original cells, may often be seen 

 imbedded in the walls of the fully-formed capillary 

 vessels. In regard to the size of the capillary vessels, 

 there is considerable variation ; some being so small, 



as only to admit globules of blood in single file ; whilst First appearance of blood- 

 others, passing directly between arteries and veins, vessels in the vascular layer 

 admit several rows at once. From the measurements of ' he 7 gem ? na ' '' bnn ? ( 



e TTT i it/f n 11- i a Fowl at the 36th hour of m- 



of Weber, Muller, and others, it appears that the cuba uon. (After Wagner.) 

 capillaries in Man vary from about ^Tn^ to T3Vo tn 



of an inch ; whilst the blood-corpuscles vary from about T ^j- 7 th to o-j V^ tn f 

 an inch in diameter. As the capillaries cannot be examined in the human 

 body until after death, and then only by means of forcible injections, these 

 measurements may be somewhat inaccurate. To the larger tubes, (which 

 may perhaps be more numerous in cold-blooded than in warm-blooded Verte- 

 brata), some would deny the term capillary; but in the sense in which that 

 word is here employed, it is strictly applicable to all those minute vessels 

 which connect the arterial and venous systems. 



478. The Size of the Capillary vessels in any part is continually under- 

 going variation ; sometimes all of them enlarging or contracting simultaneously ; 

 and one sometimes contracting, whilst others enlarge. In regard to the first 

 of these phenomena, more will be said hereafter ; the second is here noticed, 

 because it explains an occasional appearance, on which some have founded 

 their belief in the non-existence of distinct parietes to these vessels. In 

 watching the capillary circulation in any transparent part, we not unfrequently 

 see the globules of blood running into passages of the tissue, which we did not 

 perceive before ; but on a more careful examination, the observer may satisfy 

 himself that these passages existed previously, and that the fluid part of the 

 blood was transmitted through them ; the stoppage of the red particles being 

 in a great measure dependent on some partial or local impediments. The 

 compression of one of the small arteries, for instance, will generally occasion 

 an oscillation of the globules 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 former velocity and force, and flows exactly 

 into the same passages as before.* It may also be frequently observed, that 

 the rate of motion is very different in the different parts of the network ; and 

 that an entire stagnation of the current sometimes takes place in some particular 

 tube, the motion of the globules recommencing, but in an opposite direction. 

 Irregularities of this kind, however, are more frequent when the Heart's 

 action is partially interrupted ; as it usually is by the pressure to which the 



* Dr. Allen Thomson, in Cyclop, of Anat. and Phys., Art. Circulation. 

 31 



