The Centrai. Nkrvous System. 



189 



/=- k'-l\ 



(c) The fourth cranial, or trochlear nerve (n. trochlearis), arises 

 from the anterior medullary velum. The first portion of the 

 nerve is usually to be found on the lateral surface of the 

 cerebral peduncle. 



(d) The posterior medullary velum (velum medullare posterius) 

 underlies the posterior margin of the cerebellum, and extends 

 backward over the triangular space enclosed by the walls of 

 the fourth ventricle. The structure forms a chorioid plexus, 

 similar in character to that of the third ventricle but much 

 less extensive. It is commonly torn away in the preparation 

 of the brain, in which case the interior of the fourth ventricle 

 is exposed. 



(e) On the ventral surface (Fig. 54), 

 the pons forms a broad com- 

 missural band extending trans- 

 verselv across the brain and 

 upward into the supports of the 

 cerebellum. Its surface is 

 divided into two parts by a 

 median depression, the sulcus 

 hasilaris, occupied by the 

 basilar artery. 



(f) The anterior median fissure of 

 the spinal cord ends at the 

 posterior margin of the pons 

 in a faint depression, the fora- 

 men caecum. 



(g) The anterior funiculus of the 

 cord is largely replaced forwards 

 by the pyramid (pyramis), a 

 narrow band extending forward 

 on either side of the middle 

 line to the posterior margin of 

 the pons. 



(h) The trapezoid body (corpus 

 trapezoideum) lies in the angle 

 formed by the lateral margin 

 of the pyramid with the pos- 

 terior border of the pons. 



(i) The fifth cranial, or trigeminal 

 nerve (n. trigeminus), arises by 

 two roots, a larger sensory 

 root, the portio major, and a smaller motor root, the portio 

 minor. The two parts appear at the lateral border of the pons. 

 The portio major is the common trunk of the ophthalmic, maxillary 

 and mandibular nerves, the portio minor joining the mandibular, so 

 that the latter becomes a mixed nerve. The cut end of the portio 

 major mav be identified on the cranial wall and traced forward 

 in the semilunar ganglion, the latter lying in a depression at the 

 anterior ventral end of the petrosal bone. 



fU- 



FiG. 54. The rhombencephalon. 

 Ventral view (the cerebellum not 

 figured). 



c.t., trapezoid body; f.c, foramen 

 caecum; fl.c, cervical flexure; f.m.a., 

 anterior median fissure; p., pons; 

 p.c, cerebral peduncle (mesencep- 

 halon); py., pyramid. 



Ill, oculomotor nerve; IV, troch- 

 lear; VI, portio major of the trige- 

 minus; V-. portio minor; VI, 

 abducens; VII, facial; VIII, acoustic" 

 IX-XI, glossopharyngeal, vagus, and 

 spinal accessory group; XII, hypo- 

 glossal; CI, first cervical spinal. 



