32 



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



continued. One or more respiratory movements are often excited at 

 the same time that cardiac arrest is brought about. Stimulation of 

 the gill apertures and of the internal surface of the branchial chamber 

 also gives rise to cardiac inhibition, though in a much less marked 

 degree than does stimulation of the gills. 



The phenomenon of cardiac inhibition as a result of gill stimulation 

 is not confined to the eel. I have found it to be well marked in various 

 other fishes — carp, perch, rudd, and others. 



Stimulation of the skin of the head is also effective in bringing 

 about an arrest of the heart's action. The application of pressure or 

 slight friction, or a weak interrupted current, or a thermal stimulus, 

 leads readily to this result. If the skin be removed, slight stimulation 

 ceases to have any effect unless pressure be applied over the branchial 

 chamber — an inhibitory effect is then obtained by the gills being 

 affected. Severe injury of any part of the head after removal of the 

 skin may still (apart from any influence upon the gills) bring about 

 cardiac inhibition. Stimulation of the fifth nerve is followed by a 

 similar result. The administration of ether or chloroform seems as a 

 rule to prevent the occurrence of cardiac arrest as a result of slight 

 stimulation of the skin of the head ; this occurs when the quantity 

 of ether that has been used is quite insufficient to suspend the activity 

 of the medulla, or to prevent the arrest of the heart which follows gill 

 stimulation. 



Gentle compression of the animal's tail between the fingers is 

 generally followed by standstill of the heart. So also is the appli- 

 cation of electrical or chemical stimulation to the same part. The 

 inhibitory result is obtained by irritation of the integument covering 

 the caudal fin as well as of that covering the muscular part of the 

 tail. If the skin be removed gentle mechanical stimulation ceases to 

 have any effect ; electrical stimulation is apt to spread to the spinal 

 cord. The area of skin over which stimulation is effective generally 

 extends from the tip of the tail forwards about one-eighth of the 

 distance to the vent. 



Muscular movements along the animal's body are induced at the 

 same time as the cardiac standstill. 



The path along which the afferent impulses pass to the medulla is 

 the spinal cord. Section of the cord above the point of stimulation 

 prevents the occurrence of a cardiac inhibitory result. 



Direct stimulation of the spinal cord is a very powerful means of 

 arresting the heart's action, and the same result occurs at whatever 

 part the cord is stimulated. The cardiac effect is accompanied by 

 vigorous contraction of the muscles generally, including those or 

 the pectoral fins. But when an interrupted current is employed tot 

 stimulate the cord, the question may be raised whether some escape* 

 of the current to the neighbouring structures may not occur, e.g.^. 



