COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 355 



brain it will be seen to narrow to a white cord, the brachium pontis or middle 

 peduncle of the cerebellum, which passes into the substance of the cerebellum. 

 The pons is, in fact, a bridge between the two hemispheres of the cerebellum. 

 It is the ventral part of the metencephalon of which the cerebellum is the dorsal 

 portion. Immediately posterior to the brachium pontis and partly concealing 

 it is the thick root of the trigeminus nerve. On close examination this will be 

 seen to consist of a large dorsal portion, the sensory root (portio major) which 

 consists of the somatic sensory fibers of the trigeminus, and a very small ventral 

 portion, the motor root {portio minor) which contains the visceral motor fibers 

 for the muscles of mastication (masseter, temporal, digastric, etc.). Posterior 

 to the pons and of about half its width is another bundle of transverse fibers 

 the trapezoid body. Close inspection will show that the trapezoid body originates 

 from the area acustica or auditory center ; it passes toward the median line but 

 before reaching it, turns forward and disappears dorsal to the pons. The trape- 

 zoid body is the main tract which carries the auditory impulses to the more 

 anterior portions of the brain. Attached to the side of the area acustica is the 

 root of the eighth or auditory nerve. Just ventral to this and behind the root 

 of the trigeminus is the root of the facial nerve emerging through the trapezoid 

 body. In the median ventral line of the medulla is a groove, the median ventral 

 fissure. Along each side of this runs a narrow bundle of fibers; each emerges 

 dorsal to the posterior margin of the pons and proceeds straight posteriorly. 

 These two tracts are the pyramids or somatic motor tracts; they convey impulses 

 from the cerebral hemispheres to the voluntary muscles. At the place where 

 the pyramids emerge from above the pons are the roots of the sixth or abducens 

 nerves. The small root of the ninth or glossopharyngeal nerve will be found at 

 the posterior boundary of the acustic area, at the point where the restiform body 

 passes dorsal to it and about on a line with the root of the eighth nerve. The 

 equally small root of the tenth or vagus nerve lies immediately posterior to and 

 on a line with the root of the ninth nerve. Posterior to the vagus are the numer- 

 ous roots of the eleventh or spinal accessory nerve, arising in a line. The main 

 root of the accessory ascends from the spinal cord, but is probably missing in 

 the specimen. The roots of the twelfth or hypoglossal nerve emerge along the 

 lateral border of the pyramid, posterior to the preceding roots. 



Draw the ventral view of the brain, including the roots of the cranial nerves 

 as far as you have seen them. 



d) The median sagittal section: Now cut the larger half of the brain along 

 the longitudinal cerebral fissure so as to obtain an exact median sagittal section. 

 In making such a cut use a dull knife and pass it through the brain with one 

 sliding stroke. Examine the cut surface. The cerebral hemisphere forms a 

 thick roof which arches posteriorly above the diencephalon and midbrain. In 

 the cerebral hemisphere identify the section of the corpus callosum. This is an 

 obliquely placed longitudinal band of white material. Both anterior and posterior 



