APPENDIX TO THE EXPERIMENTAL SECTION. 



Dissection fop exposing 1 the Vagus nepve in the fpog. 



The pithed frog is laid upon its back on the frog plate, is freely incised 

 mesially through skin and then sternum. 



The edges of these are drawn widely apart and kept so with threads 

 pinned to the plate. Restraining connective tissue is divided close 

 to the bone, and the attachments of the pericardium are carefully re- 

 moved around the heart. 



A glass tube, 1*5 to 2 cm wide according to the size of the frog, is 

 passed down the O3sophagus as far as it will go ; this stretches the 

 neighbouring structures. 



From the angle of the jaw a somewhat deeply situated and thin 

 muscular band, composed of the petrohyal muscles, extends to the region 

 of the heart (strictly speaking, to the hyoid cartilage). 



The Vagus, dividing into its cardiac and laryngeal branches, lies 

 beneath the lower edge of this muscle, and must be carefully sought for, 

 as it is usually very small. 



The petrohyals lie above the pronounced levator anguli scapulae 

 muscle that slants down and outwards to the upper limb. 



The petrohyals are crossed by two distinct nerves. 



One, the glossopharyngeal, sweeping in a curve from the angle of the 

 jaw, passes upwards to disappear amongst the muscles of the floor of 

 the mouth. 



The other, the hypoglossal, usually piercing the levator anguli scapulae 

 curves inwards in the same direction as the first, to disappear nearer 

 the middle line. 



The Vagus must be carefully separated from the muscle for as long 

 a distance as possible. A moistened thread is passed beneath it and is 

 tied near the angle of the jaw. The nerve is cut between the jaw 

 and the ligature, and it can then be raised clear of its surroundings for 

 the application of the electrodes. 



Du Bois Reymond's compensation method fop the measupe- 

 ment of the electromotive fopce in muscle and nepve. Establish 

 the connections as shown in the figure, with one gap of the metre bridge 1 



1 Instead of the metre bridge, the zig-zag resistance, fig. 46, pg. 234, may be employed, 

 as its wire is longer and thinner, and smaller differences can be more readily detected 

 by its means. This wire is of about 20 ohms resistance and is divided into 10 parts, 

 fractions of which can be measured with a foot rule. The portions in contact with 

 the pulleys around which the wire is stretched must be omitted from the length 

 measured. 



