140 ON THE CIRCULATION OF THE BLOOD, 



the stimulus of the blood itself; and which contraction would be permanent, 

 but that the heart appears to become exhausted in a considerable degree of 

 its muscular irritability by the exertion that produces the contraction, and 

 hence speedily returns to its prior state of relaxation, exhibiting 1 that alter- 

 nating- succession of systole and diastole which constitutes pulsation.* 



In the venal system, however, we meet with even fewer proofs of muscular 

 fibre than in the arterial, and no such force of the heart as to produce pulsa- 

 tion on a pressure of the finger ; and hence, to this moment, we are in a 

 greater degree of ignorance as to the projectile power by which this system is 

 actuated. The theories that have been chiefly advanced upon the subject 

 are, first, that of a vis a tergo, or an impetus given to the blood by the arterial 

 contraction, which is supposed by its supporters to be sufficient to operate 

 through the whole length of the venal canals ; secondly, that of capillary at- 

 traction, the nature of which we explained in a former lecture ; and lastly, 

 a theory of a much more complicated kind than either, and which supposes 

 the projectile power to result jointly from the impetus communicated by the 

 heart and arteries, from the pressure of the surrounding organs, and espe- 

 cially from the elasticity of the lungs, and the play of the diaphragm, in con- 

 junction with the natural irritability of the delicate membrane that lines the 

 interior of the veins. It is unnecessary to enter into a consideration of any 

 of these theories ; for they all stand self-convicted of incompetency ; and 

 the last, which is the most operose of the whole, has been only invented to 

 supply the acknowledged inefficacy of the other two.f Whatever this projec- 

 tile power consists of, it appears to have some resemblance to that of the 

 vegetable system ; and, like many of the vessels in the latter, is assisted 

 by the artifice of numerous valves inserted in different parts of the venal 

 tubes. 



The most important process which takes place in the circulation of the 

 blood is that of its ventilation in the lungs. It is this process which consti- 

 tutes the economy of RESPIRATION, and has till of late been involved in more 

 than Cimmerian darkness. 



We see the blood conveyed to the lungs of a deep purple hue, faint and 

 exhausted by being drained in a considerable degree of its vital power, or 

 immature and unassimilated to the nature of the system ; t is about to support, 

 in consequence of its being received fresh from the lacteal trunk. We behold 

 it returned from the lungs spirited with newness of life, perfect in its con- 

 formation, more readily disposed to coagulate, and the dead purple hue trans- 

 formed into a bright scarlet. How has this wonderful change been accom- 

 plished 1 what has it parted with ] what has it received! and by what means 

 has so beneficial a barter been produced ? 



These are questions which have occupied the attention of physiologists in 

 almost all ages ; and though we have not yet attained to any thing like demon- 

 stration, or even universally acceded to any common theory, the experiments 

 of modern times hure established a variety of very important facts which 

 may ultimately lead to such a theory, and clear away the difficulties by which 

 we are still encumbered. 



These facts I shall proceed to examine into in language as familiar as I 

 can employ : I must nevertheless presume upon a general acquaintance with 

 the elementary principles and nomenclature of modern chemistry, since a 

 summary survey of zoonomy is not designed to enter into a detail of its 



Physiological experiments have sufficiently proved of late that the same alternation of contraction 

 and dilatation does not take place in the arteries in a free or natural state ; for where there is no resist- 

 ance to the flow of the blood along their canals, there is no variation in their diameter ; and that it is only 

 the pressure of the finger or some other substance against the side of an artery that produces its pulse. 

 Study of Med. ii. p. 16. Experimental Inquiry into the Nature, &c. of the Arterial Pulse, by C. H. Parry, 

 M.D. 1816. 



T It has lately been pretty clearly established, that by far the most active power in the return of the 

 blood to the heart from the veins, is the comparative vacuum which takes place in the ventricles of the 

 heart when exhausted of blood Dy the systole or alternating contraction of this organ; in consequence of 

 which, the venous blood is, as it were, sucked up into the right ventricle from the veme cavas, or 

 venous system at large. So that the heart, upon this beautiful principle of simplification, becomes alter- 

 nately a forcing and a suction pump. By its contraction it forces the blood into the arterial system, and 

 by its vacuum it sucks it up from the venous. See Study of Med. ii. p. 19, 2d edit. 1825. 



