302 ANATOMY OF THE RABBIT 



(q) The cervical portion of the sympathetic trunk lies on the 

 dorsal surface of the common carotid, and is slightly medial 

 in relation to the vagus. 



Section and stimulation of the sympathetic in the neck is one of the 

 classic demonstrations of vaso-motor action. The result of section can 

 be seen in reddening and loss of heat in the ear (vasodilatation), and 

 contraction of the pupil of the eye. Stimulation has the opposite effect 

 (vaso-constriction) . 



(r) The ramus cardiacus of the vagus (n. depressor) lies on 

 the dorsal surface of the common carotid on the medial side 

 of the sympathetic trunk, arising at the level of the pos- 

 terior margin of the thyreoid cartilage. It is an afferent 

 nerve. Its fibres are said to originate from cells in the upper 

 pole of the jugular ganglion (a mass of nerve cells forming 

 two slight swellings on the vagus nerve just before it 

 emerges from the skull). Those of the left terminate peri- 

 pherally in the arch of the aorta, those of the right in the 

 subclavian, being distributed along with other nerve fibres 

 in the cardiac plexus. 



Occurring in the rabbit as a separate nerve, the depressor is im- 

 portant experimentally. Stimulation of the proximal end in the living 

 animal produces fall of blood pressure and retardation of the heart 

 beat. The former is due to reflex action on the blood-vessels (cf . p. 64) , 

 while the latter depends upon reflex inhibition by impulses passing 

 through the vagus, as is shown by the fact that slowing of the heart 

 does not take place if the vagi also are divided. 



(s) The third and fourth cervical nerves may be traced from 

 their origin in the intervertebral foramina to the muscu- 

 lature of the neck. They encircle the basioclavicularis 

 muscle, under cover of the sternomastoideus and cleido- 

 mastoideus. 



5. Dissection of the muscles of mastication and related structures 

 of the mandible. 



(a) The masseter muscle. Origin: The zygomatic arch. In- 

 sertion: Lateral surface of the angle of the mandible (1, a), 

 also that of the ramus. Some of the most anterior fibres 

 curve round the ventral edge of the mandible and pass back 

 medial to it, covering the ventral part of the internal 



