40 



Dr. J. A. Mc William. On the [Dec. 18, 



strongly evidenced when the optic lobes and medulla are both intact 

 than when the former parts (the optic lobes) have been removed. 



That such a controlling influence is at times present in a very 

 marked degree, is evidenced by the fact that section of the vagi 

 sometimes leads to a very great change in the rapidity of the heart's 

 action. The number of beats occasionally increases from a rate of 

 15 per minute before section of the vagi, to a rate of 40 per minute 

 after section of the vagi. Changes in the degree of continued 

 controlling influence exerted by the central nervous system upon the 

 heart seem to play a very important part with regard to the after 

 effects which are manifested as consequences of reflex arrest of the 

 heart. When the cardiac action recommences after reflex inhibition 

 the beats are at first slow, but soon increase in rate and frequently 

 attain for a time a much greater rapidity of rhythm than was 

 manifested before the reflex inhibition occurred. At the same time a 

 slight increase of the force of the auricular beats may occur. These 

 changes in rate and force seem to be in very many cases due for 

 the most part to a diminution of the constant controlling influence 

 which acted upon the heart before the inhibition was caused. It 

 would seem that strong action of the medullary inhibitory centre 

 (exerted by afferent impulses) and leading to cardiac standstill, is 

 followed by a phase of markedly diminished activity of that centre — 

 a diminution which involves a relaxing' of the control exerted upon 

 the heart, and a consequent increase in the rate of the heart's beat and 

 occasionally also of the auricular force of contraction. That such 

 a diminution of the medullary controlling influence is frequently 

 the main cause of the after-acceleration following reflex inhibition, 

 is supported by a number of considerations ; among others, by the fact 

 that in many instances where such after-acceleration is well marked 

 as a result of reflex inhibition, no after-acceleration at all occurs 

 after inhibition from direct stimulation of the vagi or of the sinus. 



The state of excitement into which the medullary vagus centre is 

 thrown by strong afferent impulses (from the gills and ether parts) 

 seems to persist for a considerable time after the stimulation (of 

 the gills, &c.) is discontinued. For the heart frequently remains quies- 

 cent for half a minute or more after gill stimulation is discontinued ; 

 whereas after direct stimulation of the vagus nerve the heart 

 recommences very shortly after the stimulation is discontinued. 

 Free administration of ether or chloroform leads to complete loss 

 of reflex inhibition as well as of reflex action generally. Cardiac 

 arrest can still be readily observed as a result of direct stimulation of 

 the vagus nerve. A smaller amount of these anaesthetics induces 

 a condition much resembling that seen after removal of the optic 

 lobes. There are no voluntary movements. Reflex action upon the 

 qeart or skeletal muscles cannot be excited by very slight 



