i 3 o THE MECHANISM OF THE CIRCULATION. 



In animals with quick respiration, such as cats, rabbits, and guinea-pigs, the 

 total effect is maximal at the height of expiration, minimal at the depth of 

 inspiration. 



The Vasomotor Mechanism. 



Historical. — As early as 1733 the notion that the small arteries 

 change their calibre was put forward by Stephen Hales. 1 Among the 

 many ingenious experiments he devised is the following : — Tying a brass 

 tube into the aorta of a dog, and employing a head of pressure equal to 

 the normal aortic tension, he injected water and measured the outflow, per 

 minute, from the divided vessels of the intestine. He found that while 

 cold water diminished, hot water increased the outflow. Further, from 

 investigations on the action of such drugs as alcohol and infusion of 

 cinchona bark, he concluded that while one set of agents, by constricting 

 the vessels, lessens the outflow, another set widens the vessels and 

 increases the outflow. 



On scratching the femoral or the carotid artery with the point of a knife, 

 the local constriction of these vessels can be evoked. Likewise by chemical 

 irritation, or the application of the galvanic current, the peripheral arteries of 

 the frog can be thrown into contraction. Observations of this kind were 

 recorded by several experimentalists towards the end of the eighteenth and the 

 beginning of the nineteenth centuries. 



Hunter observed that the carotid artery of the dog gradually contracts if it 

 be exposed to the air, while Parry obtained the same result on loosely tying a 

 ligature round an artery. Mayo 2 records that the diameter of an artery is 

 markedly increased by rubbing it for the space of half a minute between the 

 finger and thumb. 



Some time after this it was recorded, and more than once, that congestion 

 of the head and neck follows after division of the cervical sympathetic nerve, 

 or excision of the superior cervical ganglion. 3 



By Krimer 4 the interesting observation was made, that the application of 

 salt to a frog's web does not, when the nerves of the part are injured, produce 

 its usual effect of dilatation of the capillaries. 



By E. H. Weber 5 the direct proposition was made, for the first time, that 

 pallor and blushing could be explained, if it were once taken for granted that 

 the calibre of the peripheral arteries could under nervous influence become 

 smaller or larger. 



The position in which the question of the contractility of the arteries stood 

 in 1836 is illustrated by the following observations of Allen Thomson : — 



" It may be remarked that the coldness and impaired nourishment common 

 in palsied limbs, the known increase or diminution of the various secretions 

 from mental emotions, and direct or sympathetic affections of the nerves 

 belonging to the glands or other secreting organs, the phenomena of blushing, 

 erection, inflammation, and the like, are all very direct and satisfactory proofs 

 that the small vessels and the capillary circulation may be influenced by 

 affections of the nerves." 6 



Physiological ideas drawn from experiments thus forestalled anatomical 

 discovery, for the muscular coat of the arteries remained undiscovered until 



1 "Statical Essays," 1733, vol. ii. 



2 Mayo, "Outlines of Human Physiology," London, 1833, p. 67. 



a See Milne-Edwards, " Lecons sur la Physiologie," Paris, 1859, tome iv. p. 207. 



4 "Physiol. Untersuch.," Leipzig, 1820. 



5 Hildehrandt's " Handbuch der Anatomie," 4th edition, 1831, pt. iii. S. 75. 



6 Todd's "Cyclopaedia of Anat. and Physiology," 1836, vol. i. pp. 672, 680. 



