OF THE FUNCTIONS OF THE CAPILLARIES. 45 



Of the, Functions of the Capillaries, 

 Note S. 



This class of vessels may be divided into two orders, performing distinct functions ; 

 1st, those capillary vessels between the terminations of the aortic arteries and the 

 commencement of the veins of the body ; and 2d, those between the termination of the 

 pulmonary arteries and veins of the same name The first of these orders is disposed, 

 in different proportions, to the compound solids of the body ; the second is distributed 

 on the surface of the air-cells of the lungs only. In the one are performed changes 

 which, render the blood unfit for the purposes of the animal economy ; in the other takes 

 place an elaborate process of an opposite nature. In the first are produced those or- 

 ganic functions which relate more directly to the noui'ishment of the frame, as digestion, 

 secretion, and nutrition ; in the second, those preparatory operations on the blood which 

 enable the sensible and contractile textures of the body to perform their offices. With- 

 out the accomplishment of the latter, the former could not be performed : for, as the 

 former depends upon the vital influence distributed to the capillaries and to their re- 

 spective organs, as well as upon its state in the sources whence it is derived, so does 

 this influence itself depend upon the operations which take place in the latter order of 

 capillaries. The importance, therefore, of these operations in the animal economy must 

 be manifest, as well as the intimate bond which unites them throughout the frame : 

 without the performance of the one class of functions, the other could not be dis- 

 charged. 



This part of the circulation the most interesting, perhaps of any, to the physiologist 

 and pathologist, without being independent of the heart's action, is the least under its 

 control ; the functions of the capillary vessels continuing to a certain extent, even after 

 the heart has ceased to contract: and, asha\e been shown by some experiments per- 

 formed in this country, in France, and in America, these actions are not limited, even 

 then, to the mere circulation of the fluid which they contain ; for under this particular 

 circumstance, they may also perform, for a short time, the functions of absorption, and 

 secretion. 



These phenomena may be readily explained when we consider two circumstances ; 

 1st, the source whence the capillaries derive their functions ; and 2d, the kind of death 

 which the animal experiences, and the order in which the different organs cease to act. 

 We cannot enter here further into this topic ; we have pointed out the way : those who 

 are interested in it will be able to pursue it ; those who are not, would profit little from 

 a lengthened explanation. 



Before we leave this subject we may notice an opinion which has been entertained 

 amongst the most eminent physiologists. This relates to the existence of subordinate 

 sets of minute and colourless arterial capillaries, each devoted to a particular function ; 

 namely, one to nutrition, another to secretion, and a third and principal set, to the 

 transmission of the red blood, which, in consequence of the functions of the former two, 

 have become possessed of venous properties. The first and second of these sets are 

 considered to be pellucid in their natural state, and, although they cannot be satisfac- 

 torily demonstrated, in this state, their existence seems to be rendered probable, if not 

 proved, by many of the phenomena of disease and by artificial injections. 



Dr. Alard has lately contended for the existence of a similar set of colourless vessels 

 connected in the same manner with the veins ; and that whilst those of the arteries 

 carry the fluids intended for the nutrition of the textures, for the secretions, and exhala- 

 tion ; these belonging to the veins perform the functions of absorption. Some of these 

 latter vessels, whose open mouths are present every where, in the most intimate tex- 

 tures of the organs, as well as on the surfaces of the great cavities, are supposed by Dr. 

 Alard to terminate in the parietes of the adjacent veins; whilst others unite and form 

 the trunks which are generally known by the name of absorbents. The discovery of 

 Dr. Fohman, of Heidelberk, of a communication of the lymphatics of the intestine with 

 the mesenteric veins, in some animals, concurs to support the proposition of Dr. Alard. 

 On this subject Dr. Hutchinson, whose physiological knowledge is of the first order, 

 has justly observed, the view of Alard that supposing the existence of minute pellu- 

 cid vessels, springing from the parietes of the small arteries; distributed to every part 

 of the body; conveying different fluids, and producing different effects, according as 

 their vital properties are modified ; having corresponding vessels, which spring from 

 the most intimate texture of the organs and surfaces of the great cavities, and umte in 



