126 SPECIAL PH YSIOL OGY 



operation yet attempted in any of these exercises. The nerve is very 

 small and lies deeply buried in the neck not far from the spinal 

 column and in close relation to other nerves, making it difficult so 

 to describe its relations that the operator will be certain when he has 

 found it. The only sure course is to test it by stimulation, and if 

 it causes a contraction of the corresponding side of the diaphragm 

 the operator may be certain he has found either the main trunk or 

 one of its three roots. 



The cutaneous incision should be on the course of the sterno- 

 mastoid muscle, just dorsal to the course of the external jugular vein. 

 The cutaneous incision should be ample, extending to the clavicle 

 at least 5 cm. long anteriorly in the rabbit and correspondingly long 

 if the dog is used. 



FIG. 65 



Phrenograph for taking tracings of the movements of the diaphragm : T, tambour joined 

 to recording tambour and fitted with a cork button (C); a glass rod passes through a slit in the 

 abdominal wall at .Fand rests against the diaphragm at A. 



Dissect through the subcutaneous tissues and separate the skin 

 flaps widely, pressing the external jugular toward the median line. 

 The superficial layer of muscles consists of the sternomastoid on 

 the median side and the cleidomastoid laterally in the rabbit (the 

 cephalohumeral in the dog). 



Divide the connective tissue that separates these two muscles and 

 pass to the deeper layer. On the median side one sees the carotid, 

 the internal jugular, and the nerves that lie in close relation to them; 

 drawing the cleidomastoid outward one exposes the roots of the 

 brachial plexus, emerging from between the deep muscles of the 

 neck and passing downward and backward toward the axilla. 



Very careful dissection of the delicate connective tissue which lies 

 over the roots of the brachial plexus will reveal a fine nerve thread 

 crossing these roots very near to the line where they first come into 

 view, and passing posteriorly it gradually draws nearer to the median 

 line as it passes under the clavicle (under the subclavian artery in 

 the dog). This nerve is the phrenic. Carefully dissect it out as near 

 the clavicle as possible, lift it gently on the nerve hook, and place it 



