KESPIRATION, AND ANIMALIZATION, 



143 



excited by the stimulus of the blood itself ; and which contraction would be 

 permanent, but that the heart appears to become exhausted in a consider- 

 able degree of its muscular irritability by the exertion that produces the 

 contraction, and hence speedily returns to its prior state of relaxation, ex- 

 hibiting that alternating succession of systole and diastole which constitutes 

 pulsation.* 



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

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

 pulsation on the 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 sys- 

 tem 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 attraction, the nature of which we explained in a former lec- 

 ture ; and lastly, a theory of a much more complicated kind than either, and 

 which supposes the projectile power to result jointly from the impetus com- 

 municated by the heart ?ind arteries, from the pressure of the surrounding 

 organs, and especially from the elasticity of the lungs, and the play of the 

 diaphragm, in conjunction with the natural irritability of the delicate mem- 

 brane 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.j 

 Whatever this projectile power consists of, it appears to have some resem- 

 blance 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 con- 

 stitutes the economy of REsriRATioN, 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 it is about to sup- 

 port, 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 

 conformation, more readily disposed to coagulate, and the dead purple hue 

 transformed into a bright scarlet. How has this wonderful change been 

 accomphshed ? 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 



* Physiologfical experiments have sufBciently 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 resistance 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 ticger or some other substance against 

 the side of an artery that produces its pulse. Stud, of Med. ii. p. 16. Experimental Inqaiiy 

 into the Nature, &c. of the Arterial Pulse, by C. H. Parry, M. 1816. 



t It has lately been pretty clearly established, that by far the most active power in the re- 

 turn of the blood to the heart from the veins is the comparative vacuum which takes place ia 

 the ventricles of the heart, when exhausted of blood by the systolflf or alternating contraction 

 of this organ , in ronsequence of which, the venous blood is, as it were, sucked up into the 

 right ventricle from the ven» cavae, or ven(»us system at lai^e. So that the heart, upon this 

 beautiful principle of simplification, becomes alternately a Arcing and a suction piuup. By 

 its contraction it forces the blood into the arterial system, and by its vaCuuin it swcKs it up 

 from the venous. «ee Stud, of Med. ii. p, 19. 2d edit. 1826^ 



19 



