DISSECTION OF THE DOG 211 



N. trigeminus. — The trigeminal is the largest of the cerebral nerves and 

 arises by two roots from the lateral part of the pons. Of the roots, the larger 

 (portio major) is sensory in function ; while the much smaller medial root 

 (portio minor) is motor. 



N. abdttcens. — The abducent nerve has its origin from the most anterior 

 part of the groove forming the lateral limit of the pyramid of the medulla 

 oblongata. 



N. facialis et x. acusticus. — The facial and acoustic nerves arise 

 together from the lateral extremity of the corpus trapezoideum. The facial 

 is the more medial, and slightly the smaller. 



N. glossopharyngeus, n. vagus, et N. accessorius. — From the lateral 

 border of the ventral surface of the medulla, a series of nerve rootlets 

 take origin. These join to form the glosso-pharyngeal, vagus, and accessory 

 nerves. The accessory nerve, in addition to its medullary roots, contains 

 fibres derived from the cervical part of the spinal cord. The spinal roots of 

 the accessory unite as a single cord, which enters the cranium by the foramen 

 magnum and joins the medullary roots to form the complete nerve. 



N. hypoglossus. — The hypoglossal nerve arises by several rootlets from 

 the aboral portion of the groove lateral to the pyramid. 



Dissection. — As a preliminary to the examination of the surface of the 

 cerebral hemispheres, they must be carefully denuded of the covering 

 formed by the pia and arachnoid. Since the whole of their surface 

 cannot be examined without the separation of the two hemispheres 

 from each other, it is well to procure two brains. One of these should 

 be left intact ; while the cerebellum, medulla oblongata; and pons of 

 the other should be separated from the cerebrum by a transverse in- 

 cision made just in front of the pons. The two hemispheres of this 

 specimen may now be isolated by an incision in the plane of the great 

 longitudinal fissure. 



HEMiSPHiERiA cerebri.— The two cerebral hemispheres form the greater 

 part of the bulk of the brain. Each is flattened laterally in a certain degree, 

 and so possesses two surfaces — lateral and medial. The lateral surface, for 

 the main part convex, is applied to the walls of the cranium. The medial 

 surface, on the contrary, is flattened, and much of it faces the corresponding 

 surface of the opposite hemisphere ; but the more aboral portion, faintly 

 concave, joins the rest at an angle and is separated from the cerebellum by 

 the tentorium only. 



If, in the intact specimen, the hemispheres are separated as far as possible 

 by opening the longitudinal fissure between them, it will be found that they 

 are connected by a broad commissural band, the corpus callosum. It will 

 be observed, moreover, that the width of the commissure is not as great as 



p2 



