LESSON XXII 



THE CIRCULATION (Continued) 



THE VASOMOTOR ACTION OF THE CERVICAL SYMPATHETIC, 

 DEPRESSOR, AND SCIATIC NERVES 



1. The Cervical Sympathetic Nerve. Etherize a rabbit and main- 

 tain the anesthesia throughout the following experiments: Perform 

 tracheotomy. Make an incision along the inner border of the sterno- 

 cleidomastoid muscle, and retract this muscle laterally outward. Do 

 not disturb the relationship of these parts by dissection. Identify the 

 carotid artery and the large vagus nerve right neighboring. In addi- 

 tion, identify two delicate nerves, one white and one gray in color. 

 The former are the depressor fibers of the vagus which pursue a separate 

 course in the rabbit, and the latter, the sympathetic fibers uniting the 

 inferior and superior cervical ganglia of this sys- 

 tem. Place each in a loose silk ligature. Shift 

 the head of the animal so that the ear on the 

 side operated on may be placed in a position for 

 inspection. Hold an incandescent lamp behind 

 it. Identify the central artery of the ear and 

 the lateral venous collecting channels. Arrange 

 the electric apparatus for stimulation with a 

 quickly interrupted current of very moderate 

 intensity. Gently raise the cervical sympath- 

 etic nerve from the wound and place it in 

 small shielded electrodes. Stimulate for a 

 period of about fifteen to twenty seconds. Note FIG. 75. SHIELDED 

 that after an appreciable latent period the (Ha r V a^Appar^us Co.) 

 central artery becomes less conspicuous and 



eventually disappears altogether. Discontinue the stimulation and 

 allow circulatory conditions to become normal again. What conclusions 

 may be drawn from this experiment regarding the vasomotor action of 

 this nerve? 



Raise the cervical sympathetic nerve and cut it between two liga- 

 tures. Inspect the blood-vessels of this ear. Note that its vascularity 

 is now much greater than before and exceeds that of the opposite organ. 

 It also possesses a higher temperature. Obviously, the division of this 

 nerve has given rise to a relaxation of the blood-vessels of the correspond- 

 ing ear. Stimulate the distal end of the divided nerve and repeat the 

 observations made previously (Fig. 76). 



It might be well to allude at this time to the influence of this nerve 

 upon the size of the pupil, although the student cannot be expected as 

 yet to be familiar with this particular reaction. Since the cervical 



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