THE NERVOUS SYSTEM 1 25 



not to cut the mandibular nerve (branch a) which Hes close 

 along the medial side of the ascending ramus. Beginning 

 at the cut edge of the ascending ramus free it from the muscles, 

 which are attached to its medial side, until it can be entirely 

 removed. Locate the mandibular nerve (branch a, or third 

 of V) , which enters the mandible near the anterior margin of the 

 transected ascending ramus. Then very carefully remove 

 all the muscle tissue in the orbit ventrad to the eyeball and 

 expose the floor of the orbit and the cranial nerves laterad 

 to the optic nerve. The largest of these, which passes craniad 

 across the floor of the orbit, and into the maxillary region 

 continuing as the supraorbital nerve through the supraorbital 

 foramen to the face is the maxillary nerve (branch b, or second 

 of the V). It emerges from the cranium through the foramen 

 rotundum and gives off numerous small branches to the 

 structures in the orbit, including a long, slender branch (the 

 palpebral) which leaves it at the foramen rotundum and passes 

 to the lower eyelid. The ophthalmic nerve (branch c, or first 

 of V) emerges just dorsomediad to the maxillary nerve, through 

 the orbital fissure (sphenoidal sinus, lacerum anterus foramen) . 



B. THE BRAIN 



(Figs. 32, S3) 



The brain and spinal cord of the rabbit are protected by a 

 comparatively tough three-layered membrane, the meninges. 

 The three membranous layers of the meninges are the dura 

 mater, which is the toughest of the three and lines the internal 

 surface of the cranium and vertebral canal; the pia mater, 

 which is a delicate membrane and intimately covers the brain 

 and spinal cord; and the arachnoid, a loose web-like membrane 

 which connects the dura and pia mater. The brain is relatively 

 smaller than the spinal cord, and the cerebral hemispheres 

 are practically free from convolutions. The cerebellum is 

 well developed and has a queer stalked structure, the floculus 

 or parafloculus, on the lateral side of each hemisphere. 



