240 Anatomy of the Rabbit. 



and th6 internal jugular vein. It gives off the n. laryngeus 

 superior to the larynx, the latter passing to the dorsal side 

 of the cornmon carotid artery. 



(p) The ramus descendens of the twelfth cranial or hypo- 

 glossal nerve crosses the root of the vagus from a lateral 

 to a medial position. It passes backward on the ventral 

 surface of the artery, and is chiefly distinguishable by its 

 branches to the sternohyoideus and related muscles. 



(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. 



(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. It arises at the level of the posterior 

 margin of the thyreoid cartilage. 



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

 important 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 a reflex action on the bloodvessels (cf. p. 62), 

 while, the latter depends upon reflex stimulation of the vagus, since 

 slowing of the heart does not take place if the vagi are also divided. 



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

 from their origin in the intervertebral foramina to the 

 musculature of the neck. They encircle the basioclavicularis 

 muscle, under cover of the sterndmasLoideus and 

 cleidomastoideus. 

 5. Dissection of the muscks of mastication and related structures 

 of the mandible. 



(a) The masseter muscle. Origin: The entire surface of the 

 zygomatic arch; tendinous from its anterior angle (spina 

 masseterica) . Insertion: Lateral surface of the angle of the 

 mandible (i, a). 



The orbital structures should be freed from the zygomatic arch by 

 passing a knife along its dorsal margin. The zygomatic arch may then 

 be divided anteriorly and posteriorly and rernoved, together with the 

 whole insertion of the masseter muscle. 



(b) The temporalis is a slender, somewhat triangular muscle 

 arising from the reduced temporal fossa (siilcus temporalis) 

 of the skull and inserted by a long stout tendon on the 



