612 THE BRAIN. 



This ascending root differs from the descending root in not having con- 

 spicuously attached to it any collection of nerve-cells ; in this respect it- 

 resembles the ascending root of the glosso-pharyngeal, and we may add part 

 of the posterior root of an ordinary spinal nerve, the fibres of which, as we 

 have seen, pass into the gray matter without being obviously connected with 

 nerve-cells. In its lower part at least it consists of extremely fine fibres, and 

 indeed looks very much like a continuation of the bulb of the marginal 

 (Lissauer's) zone of the spinal cord. 



535. The fourth or trochlear nerve. The nucleus of this nerve (Fig. 

 138, IV.) is a column of somewhat large multipolar cells on each side of the 

 median line below the aqueduct (Fig. 136, IV. n.), reaching from the level 

 of the junction of the anterior and posterior corpora quadrigemina to the 

 hinder level of the latter body. 



The root, starting from the lateral surface of the nucleus, does not take 

 at first a ventral direction, but sweeps laterally and dorsally in the outer 

 layers of the central gray matter (Fig. 136), and so curving round to the 

 dorsal surfaces reach the valve of Vieussens, where in the median line it 

 decussates with its fellow in the substance of the valve; such a decussatiori 

 at a distance from the nucleus of origin is exceptional in the cranial nerves. 

 Leaving the surface of the brain in the valve, it takes a superficial course, 

 curving (Fig. 131, B) laterally and ventrally, and makes its appearance in 

 a ventral view of the brain at the front edge of the pons, on the lateral edge 

 of the crus (Fig. 131, C). 



536. The third or oeulo-motor nerve. The nucleus of this nerve (Fig. 

 138, ///., 137, III. n.) is a column of, for the most part, fairly large multip- 

 olar cells lying on each side close to the median line, in the gray matter of 

 the central canal, just dorsal to a bundle of fibres which we shall speak of 

 as the longitudinal posterior bundle; it reaches from the level of the pos- 

 terior commissure in the third ventricle to the level of the junction of the 

 anterior and posterior corpora quadrigemina. In a section taken through 

 its middle (Fig. 137) the nucleus is seen to give off fibres which run verti- 

 cally toward the ventral surface, traversing the tegmentum and a body (En.) 



having been cut away. The nuclei are represented as if seen through transparent material. On 

 the right side, the corpus striatum and optic thalamus have been cut away horizontally to some 

 little depth in order to show their internal structure. L. lateral, E. P. external posterior, and M. 

 P. median posterior column of the cord; I. P. inferior peduncle, S. P. superior peduncle, and P. 

 middle peduncle of the cerebellum, all cut across. The dotted curved lines, upper and lower, on 

 the right half of the figure to which the dotted line P. V. outside the figure points, mark the 

 upper and lower boundaries of the pons on the ventral aspect. The outline of the fourth ven- 

 tricle is shown by a bold thick line. In the floor of the ventricle are shown, on the right half: 

 fp. fovea posterior ; Th. trigonum hypoglossi ; T. ac. trigonum acusticum ; e. t. eminentia teres : 

 . m. stride medullares on acusticee ; /. a. fovea anterior ; 1. c. locus cseruleus; I. g. valve of Vieus- 

 sens; Qp. posterior, and Qa. anterior corpus quadrigeminum ; Pg. pineal gland ; Nr. the outline 

 of the red nucleus; 3, the third ventricle, in which C. indicates the middle or soft commissure; 

 F. p. a. the pillars of the fornix, behind which is indicated in the cavity of the third ventricle the 

 hollow of the infundibulum ; C. C. g. the genu of the corpus callosum, between which and the 

 fornix the cavity often called the fifth ventricle is indicated; F. portion of convolution of frontal 

 hemisphere cut across. On the left side are shown : C. S. corpus striatum ; 0. T. optic thalamus ; 

 Pv pulvinar- T a. tuberculum anterius ; ch. s. choroidal sulcus marking the place of reflection 

 of the choroidal plexus. On the right side are exposed : JV. C. head of, Nc. end of tail of nucleus 

 caudatus ; Cip'., dp", the two parts of the globus pallidus, and Pt. putamen of the nucleus le: 

 cularis; N. a. anterior nucleus ; N. med. medium nucleus, N. lat. lateral nucleus, and Pv'. pulvinar 

 of the optic thalamus; da. front limb, Cig. knee or genu, Cip. hind limb of internal capsule ; 

 external capsule ; Cl. claustrum. The numerals III. to XII. indicate the nuclei of the respect 

 cranial nerves, all shown on the left side with the exception of the accessory vago-glosso-pharyn- 

 geal IX.. X., IX., which to avoid confusion is placed on the right side. V. is the motor nuc 

 of the fifth nerve with the descending root,F. a. the sensory nucleus of the same with the long 

 ascending root VIII. m. median nucleus, VIII. I. lateral nucleus of the auditory nerve : n. a. nu- 

 cleus ambiguus. The ascending root of the ninth nerve is seen at the hind end of the combined 

 nucleus of IX., X., XI. 



