ON. THE MOTION OF THE BLOOD. 81 



felt by the hand to expand during relaxation. He accounts, 

 however, for the full dilatation of the heart upon another prin- 

 ciple, upon which it will be impossible to enter at lengh before 

 the next section. 



(E) Most Physiologists grant to the capillaries irritability, to- 

 nicity, or organic contractility ; but some deny that arteries pos- 

 sess muscular properties. Bichat's objections are, the absence 

 of contraction on the application of stimuli to them, the much 

 greater resistance of the middle coat to a distending force than 

 of muscular parts, and, lastly, the difference of the changes 

 which it and muscles undergo both spontaneously and by the 

 action of other substances.* Berzelius has multiplied the latter 

 description of proofs, f However this may be, they have cer- 

 tainly vital powers of contraction as fully as any parts of the 

 body. This appears in their various degrees of local dilatation 

 and contraction, under inflammation, passions of the mind, &c. : 

 and if the capillaries alone are allowed to possess organic contrac- 

 tility, it is impossible to say in which point of the arterial track 

 it begins. 



Dr. Parry has instituted a number of experiments upon this 

 question. After exactly ascertaining the circumference of arte- 

 ries in animals, he killed them and again measured the circum- 

 ference ; and after the lapse of many hours, — when life must have 

 been perfectly extinguished, he measured the circumference a 

 third time. Immediately after death, the circumference was' 

 found greatly diminished, and on the third examination, it had 

 increased again. The first contraction arose from the absence of 

 the blood which distended the vessel and antagonised its efforts 

 to contract, and it was evidently muscular, or to speak more 

 correctly, organic, contraction, because, when vitality had ceased 

 and this kind of contraction could no longer take place, the 

 vessel was, on the third examination, always found enlarged. 



The forced state of distention in arteries was proved by the 



* Aiiatomie Generate. T. ii. f Animal Chemistry, p. 25. 



G 



