46 APPENDIX. 



larger tubes, forming 1 in some instances long continuous canals, denominated absorbents, 

 in others running to be inserted into veins, is one which is. qualified to explain, more 

 plausibly than any other the mechanism of the distribution of the fluids for the pur- 

 poses of the organic functions ; and is, besides, capable of obviating the difficulties 

 which have been presented by the diversity of the results of the experiments of Hunter, 

 Magendie, Broclie and others, relative to the mechanism of absorption*. (See the Note 

 on Jibsorptwn.} 



Of the Veins. 

 Note T. 



1. As to the precise way in which the veins commence, opinions have been various. 

 At the place where the capillaries change from arteries into veins, there appears to be 

 no reason to suppose the existence of either interspace, or vesicular or spongy struc- 

 ture. The inflected canal of the artery seems to be continuous with the vein. Whilst, 

 however, this conformation is allowed by nearly all, some consider, with Mr. Ribes, that 

 the veins have another commencement in addition to this ; and that a certain proportion 

 of their roots commence in open mouths or in the pores or areolse of the luminous tis- 

 sues, and in the substance of the organs. Others also suppose with M. Alard, (See the 

 preceding nvte,) that some of their roots commence in pellucid lymphatic absorbents. 



The structure of the erectile tissues, as the penis, the clitoris, the spleen, &c. seems 

 to support the opinion of Ribes, (which is also that of Meckel, who farther sup- 

 poses that one cause of the difference of the appearance and functions of organs may 

 be ascribed to the extent to which the veins originate in particular manner for which he 

 contends. 



The views of M. Alard derive their chief support from the phenomena connected 

 with absorption, but, although they appear probable, they cannot be fully demon- 

 strated. 



The veins receive but a small proportion of nerves, and these are chiefly from the 

 ganglia. The nerves supplying the pulmonary veins come principally from the anterior 

 pulmonary plexus. 



2. '1 '/it functions of the veins are 1st, to bring back the blood from the capillaries to 

 the heart : 2d, to remove and assist in the assimilation of fluids, which are absorbed by 

 the lacteals and absorbents; and 3d, in certain situations, and under certain circumstances 

 to co-operate in the function of absorption. 



Thejirst of the operations is performed by means of the vital action with which the 

 veins are endowed, assisted by the vis a tergo proceeding from the vital action of the 

 capillaries, by the contraction of the surrounding muscles viewed in connexion with 

 the direction of the valves, with which they are provided, and by the active dilation 

 of ttie cavities of the heart which derives the blood from the venous trunks. 



The third action of the veins, or absorption, seems to be proved by the researches 

 already detailed. (See Absorption in the Jlppendix.} The venous radicles, either im- 

 mediately or mediately, seize the absorbed materials, and convey them into the current 

 of the circulation. This seems to be a vital or organic action, which is probably assist- 

 ed, in some parts of the body, and under certain circumstances, by the physical property 



* Admitting fully the justness of Dr. Hutchinson's remarks, we must observe that, 

 the existence of the sets of capillaries here contended for by Dr. Alard, is not proved 

 demonstratively. Indeed, we possess this species of proof in favour only of one set of 

 capillaries, namely, those winch constitute the termination of the arteries and com- 

 mencement of veins. We know that secretion, nutrition, and absorption, are func- 

 tions of capillary vessels. This has always been granted from the time of Hippocrates ; 

 but there have been various instruments allotted to the process, some physiologists in- 

 sist, with Dr. Alard, upon the existence ol subordinate sets of capillaries allotted to 

 each function, whilst others contend, with M. Richerand, that they take place through 

 the medium of lateral pores in those capillaries which communicate directly between 

 the arteries and veins. These veins will come under consideration in the notes in this 

 Appendix on Secretion and Nutrition* 



