116 



Prof. J. A. MacWilliam. On the 



[Oct. 24, 



marked, and in 10 or 15 minutes relaxation is usually complete, even 

 in a thick-walled tube like the carotid of the ox. 



Graphic records were made of the action of ammonia vapour by an 

 arrangement similar to that used with the sulphocyanide solution. 

 Fig. 2 shows the effect upon a transversely cut strip of the wall of a 

 contracted artery, and upon a recently excised skeletal muscle (frog's 

 gastrocnemius). The upward movement of both levers indicates relaxa- 

 tion — very extensive in the case of the artery. The ammonia vapour 

 was applied at the moment of the first time signal. 



Fig. 2. 



It is to be noted that when a strip of artery is suspended in this 

 way it often contracts slightly prior to the application of any reagent 

 — apparently a response to the stimulus of stretching or of compression 

 of its ends by the clamps which hold it, or a combination of these 

 causes. 



Heating. — Keeping an artery at about body temperature for a 

 number of hours (e.g., 24) causes the contraction to pass off, as has 

 already been stated. 



Heating to about 50 — 55° C. induces complete relaxation in a few 

 minutes. The effects of heat will be described in detail later. 



Kneading, Rubbing, Stretching. — More or less extensive relaxation of 

 a contracted artery can be speedily induced by kneading or rolling the 

 vessel between the fingers for a minute ; the wall becomes much less 

 firm, and the lumen opens up markedly— e.g., from a diameter of 

 1 -5 mm. to twice that size or more ; later it may again contract to 

 some extent. 



Mayo* observed that an artery exposed in a living animal became 

 * ' Outlines of Physiology,' London, 1837. 



