382 OF THE CIRCULATION OF THE BLOOD. 



to a mechanical impediment, resulting from the contracted state of the lungs 

 in such cases, this has heen clearly proved not to be the fact, by causing 

 animals to breathe a gas destitute of oxygen, so as to cause Asphyxia in a 

 different manner ; the same stagnation results as in the other case. The 

 influence of the prolonged application of cold to a part, may be quoted in 

 support of the same general proposition; for, although the calibre of the 

 vessels may be diminished by this agent, yet their contraction is not sufficient 

 to account for that complete cessation of the flow of blood through them which 

 is well known to occur, and to terminate in the loss of their vitality. 



509. Many of the facts which indicate the influence of the Capillaries on 

 the amount and rapidity of the circulation through them, have been already 

 adverted to. It is a general principle, unquestioned by any physiologists, 

 and embodied in the ancient aphorism Ubi stimulus ibi fluxus, that when 

 there is any local excitement to the processes of Nutrition, Secretion, &c., a 

 determination of blood towards the part speedily takes place, and the motion 

 of blood through it is increased in rapidity ; and although it might be urged 

 that this increased determination may not be the effect, but the cause, of the 

 increased local action, such an opinion could not be sustained without many 

 inconsistencies with known facts. For it is known that such local determina- 

 tions may take place, not only as a part of the regular phenomena of growth 

 and development (as in the case of the entire genital system at the time of 

 puberty and of periodical heat, the uterus after conception, and the mammae 

 after parturition), but also as a consequence of a strictly local cause. Thus, 

 the student is well aware that after several hours' close application, there is 

 commonly an increased determination of blood to the brain, causing a sense of 

 oppression, a feeling of heat, and frequently a diminished action in other parts ; 

 and, again, when the capillary circulation is being examined under the micro- 

 scope, it is seen to be quickened by moderate stimuli, and equally retarded by 

 depressing agents. All these facts harmonize completely with the phenomena, 

 which are yet more striking in the lower classes of organized beings, and 

 which are evidently the results of the same laws. 



510. If the phenomena which have been here brought together be con- 

 sidered as establishing the existence, in all classes of beings possessing a 

 circulating apparatus, of a Capillary power, which affords a necessary con- v 

 dition for the movement of the nutritious fluid through those parts in which it ^ 

 comes into more immediate relation with the solids, the question still remains ' j 

 open, as to its nature. That the Capillaries possess a contractile power, far - 

 higher in degree than that of the large Arteries, and more easily excited than * 

 that of the smaller, appears scarcely to admit of doubt ; though to what it is " 

 due may be reasonably questioned. It has been recently asserted by Schwann, 

 that they possess the same kind of fibrous tissue in their walls as do the large 

 vessels : and this cannot be regarded as improbable. It is not possible, how- 

 ever, that their contractility could have any influence in aiding the continuous 

 motion of blood through them ; unless it were exercised in a very different 

 manner from that of which observation affords us evidence. For, when we 

 are microscopically examining the Capillary circulation of any part, it is at 

 once seen that the vessels present no obvious movement ; and that the stream, 

 now rendered continuous by the elasticity of the arteries, passes through them 

 as through unelastic tubes. The only method in which the contractility of 

 the Capillaries could produce a regular influence on the current of blood, 

 would be an alternate contraction and dilatation, or a peristaltic movement ; 

 and of neither of these can the least traces be discerned. Hence we should 

 altogether dismiss from our minds the idea of any mechanical assistance, 

 afforded by the action of the Capillaries, to the movement of the blood. 

 That the contractile coat of the Capillaries has for its office, to regulate the 



