CIRCULATION. 



671 



a great measure dependent on partial or local 

 impediments. The compression of one of the 

 small arteries, for instance, will frequently, 

 after causing oscillation of the globules of the 

 blood in the smallest capillaries, be followed 

 by the disappearance of some of them ; but 

 in a very short time, or when the obstruction 

 is removed, the blood regains its former velo- 

 city and force, and flows into exactly the same 

 passages as before. 



The notion that the smaller vessels are con- 

 tinuous with the smaller lymphatics, and more 

 especially with the excretory ducts of glands, 

 seems to be fully disproved by the accurate 

 researches of Malpighi, Mascagni, Panizza, 

 Miiller, and Weber, which have shewn that 

 the lymphatic vessels originate at all parts of 

 the body by a plexus of tubes every where 

 closed, and that the excretory ducts of secre- 

 tory organs begin always by shut ends. 



We believe it to be satisfactorily shewn that 

 in the whitest of the textures (with the excep- 

 tion perhaps of the cornea and crystalline lens), 

 there is no necessity for the supposition of 

 vessels admitting the fluid parts only of the 

 blood, or of serous vessels, as they have been 

 termed ; and that in all of them there exist 

 small bloodvessels which admit very fine rows 

 of globules in their accustomed proportion to 

 the fluid part of the blood : for many textures 

 which appear perfectly white or colourless, or 

 only slightly yellow when viewed with the 

 naked eye, are found, when examined with the 

 microscope, to have small vessels carrying blood 

 globules through them. Spallanzani and others 

 shewed that very small vessels taken singly or 

 seen in very thin layers have almost no per- 

 ceptible colour ; and it is a well known fact 

 that, in what are called the red textures, the 

 colour (as of muscle for instance) is not ex- 

 clusively dependent upon the quantity of red 

 blood in them. It is difficult, indeed, to con- 

 ceive how the circulation of the blood could 

 be carried on at all, or how the red particles 

 of the blood could ever be returned to the 

 heart were the globules to be retained in the 

 larger vessels, and all the white textures to 

 admit only the fluid parts of the blood. 



In adopting the opinion that the arteries 

 terminate always by direct continuity of tube 

 in the veins, and that no other visible passages 

 are connected with the minute vessels, we 

 must suppose that the various interchanges of 

 materials occurring between the blood and the 

 organized textures or foreign matters, as in nu- 

 trition, secretion, respiration, transpiration, &c. 

 must take place by some process of organic 

 transudation through invisible apertures of the 

 minute vessels. 



b. Properties of the capillary vessels and in- 

 fluence on the circulation. From the expe- 

 riments already referred to, it is apparent that 

 the smaller arteries, so long as they can be 

 distinguished from other vessels, are capable 

 of being excited to contraction by the appli- 

 cation of a stimulus ; but we have no means 

 of shewing this with regard to the minutest 

 capillary vessels, because we can scarcely apply 

 any stimulation to them without affect in some 



of the smaller arteries at the same time. When 

 it is said, for example, that the capillary ves- 

 sels are irritable, because the application of 

 ammonia or spirits of wine causes them to 

 become smaller, it is difficult to determine 

 how far this appearance of diminished size in 

 the capillaries depends on their receiving less 

 blood, in consequence of the contraction of the 

 small arteries leading to them or upon the less 

 size of these vessels themselves. In the expe- 

 riments of Dr. Thomson and others, however, 

 the application of salt and other stimuli ex- 

 citing inflammation have appeared to dilate 

 even the smallest capillary vessels, and such 

 a dilatation can scarcely be considered as in- 

 dicating any thing else than a less power of 

 resistance in these vessels ; and when the ap- 

 plication of ammonia or spirit of wine restores 

 such dilated capillaries to their natural con- 

 dition, we do not see that any other natural 

 inference can be drawn from this fact than that 

 the capillaries have been contracted by the 

 influence of these stimuli ; for the contraction 

 of the small arteries alone, although it might 

 restore the lost velocity of the blood, would not 

 diminish the capillaries to their former size. 

 This general diminution of size ought how- 

 ever to be carefully distinguished from the 

 more marked and local contractions of true 

 arteries. 



The velocity of the blood is quite uniform 

 in the capillaries of the adult animal in the 

 natural condition of the circulation. There is 

 reason to believe the capillary vessels to be 

 highly elastic, and to have the effect of com- 

 pleting the change which is begun by the 

 arteries, viz. that of equalizing the force of the 

 heart transmitted through the blood. We do 

 not, in observing attentively the capillary 

 vessels, ever perceive any motions of alter- 

 nate dilatation and contraction of their sides. 

 The blood flows through them as through 

 small glass tubes; and if they act by other 

 powers than by their elasticity alone, this 

 action must be of so slow a kind as not to be 

 perceptible. There can be no doubt that any 

 action of contraction occurring in the capillary 

 vessels, whether alternating with dilatation or 

 not, could have no effect excepting that of ob- 

 structing the passage of blood through them. 

 It would act upon the contents of the arterial 

 system much in the same way as the dimi- 

 nution of the aperture at the end of a rigid 

 tube would affect the flow of fluid through 

 it, that is, either a less quantity of blood would 

 pass through the capillary vessels in conse- 

 quence of their less size, or a greater portion 

 of the heart's force would be expended in di- 

 lating these vessels to a sufficient extent. 



The principal reasons which we feel inclined 

 to adduce for believing that the heart's action 

 is continued onwards through the capillaries, 

 and is sufficient to return the blood through 

 the veins back as far as the heart itself, are the 

 following: 1. That in an animal recently 

 killed a very small force only is requisite to 

 cause bland fluids to follow the course of the 

 blood, provided the injection be made before 

 the tonic contraction has had time to constrict. 



