THE MOTION OF THE BLOOD. 187 



They do not incessantly dilate and contract to any amount, as 

 many imagine. They lengthen and become tortuous, so that John 

 Hunter says, " instead of the term diastole it should rather be 

 called the elongated state." m Dr. Parry, on the most careful 

 examination, could never discover the least dilatation in them 

 during the systole of the ventricle when the pulse is felt. Dr. 

 Hastings declares he has seen it, as does Magendie in the case of 

 the aorta and carotid of the horse ; but from the number and 

 accuracy of Dr. Parry's experiments, I incline to believe it does 

 not occur in the ordinary undisturbed state of the circulation to 

 any extent. Sir David Barry plunged his arm into the thorax of a 

 horse, and found the aorta constantly full, nearly to bursting, not 

 perceptibly varying in distension for an instant, though he held it 

 during five minutes and examined it afterwards again ; while at 

 every expiration the cava was so empty as to feel only like a 

 flaccid thin membrane, n The fact of a continued stream occur- 

 ring from a wounded artery, only augmented at each pulsation of 

 the heart, is thought by Magendie to prove that the arteries assist 

 in propelling the blood : but an opening takes off the resistance 

 to its course so considerably that the vessel cannot but contract 

 between the impulses of the heart. 



Although the blood is constantly streaming onwards, the pulse 

 is felt only when arteries are more or less compressed ; under 

 which circumstance, the motion of the blood onwards, by the 

 impulse of a fresh portion from the left ventricle, is impeded : 

 and this effort of the fluid against the obstructing cause gives the 

 sensation called the pulse P, which follows the stroke of the heart 

 successively later throughout the arterial system, though the 

 interval is in general too minute to be appreciated. Sir D. Barry 

 found no pulsation in the aorta of the horse unless he compressed 

 it violently. 







m On the Blood, p. 175. 



n Dissertation sur le Passage du Sang a travers le Cceur. Paris, 1 827. p. 78. 



Also, Annales des Sciences Nalurelles, Juin, 1827. 



Journal de Physiologie, t. i. p. 110. 



p An Experimental Enquiry into the Nature, Causes, and Varieties of the Ar- 

 terial Pulse, &c., by Caleb Hillier Parry, M.D. F.R.S. 1816. Likewise a 

 second work, entitled, Additional Experiments on the Arteries of warm-blooded 

 Animals, &c., by Chas. Hen. Parry, M.D. F.R.S. 1819. the latter displays as 

 much talent and learning as the former of originality. Dr. Young, in a Croonian 

 lecture, highly worth perusal, on the functions of the heart and blood-vessels, 

 reasons to prove that the muscular power of arteries has very little effect in pro- 

 pelling the blood. Phil. Trans. 1809. 



