796 THE NERVOUS SYSTEM 



form bodies, and each is the superficial indication of the inferior ohvary nucleus. 



The glosso -pharyngeal, the vagus (pneumogastric), and the spinal accessory 

 cranial neives are attached along the lateral aspect of the medulla oblongata in 

 line with the facial nerve and between the olive and the restiform body. The 

 spinal accessory, purely motor, is assembled from a series of rootlets which 

 emerge from the lateral aspect of the first three or four cervical segments of the 

 spinal cord, as well as from the medulla. It becomes fully formed before reaching 

 the level of the olive, and passes lateralward in company with the vagus and fur- 

 ther on joins the latter in part. The root filaments of the vagus and glosso- 

 phar3'ngeal are arranged in a continuous series, and, if severed near the surface of 

 the medulla, those belonging to the one nerve are difficult to distinguish from 

 those belonging to the other. Both of these are mixed motor and sensory. 



The hypoglossal, purely motor, emerges as a series of rootlets between the 

 pyramid and the olive. Thus it arises nearer the mid-line, and in line with the 

 abducens, trochlear, and oculomotor. 



If the occipital lobes be lifted from the superior surface of the cerebellum and 

 the tentorium cerebelli removed, the quadrigeminate bodies of the mid-brain or 

 mesencephalon may be observed. These are situated above the cerebral pedun- 

 cles, in the region of the ventral appearance of the oculomotor and trochlear 

 nerves. Resting upon the superior pair of the quadrigeminate bodies [colliculi 

 superiores] is the epiphysis or pineal body, and just anterior to this is the cavity of 

 the third ventricle, bounded laterally by the thalami and roofed over by the tela 

 chorioidea of the third ventricle (velum interpositum). 



By separating the inferior margin of the cerebellum from the dorsal surface of 

 the medulla oblongata the lower portion of the fourth ventricle (rhomboid fossa) 

 may be seen. The cisterna cerebello-meduUaris, the subarachnoid space in this 

 region, is occupied in part by a thickening of the arachnoid. This is continuous 

 with the tela chorioidea (ligula) and chorioid plexus of the fourth ventricle. The 

 former roofs over the lower portion of the fourth ventricle, and, passing through it 

 in the medial line, is the lymph passage, the foramen of Magendie, by which the 

 cavity of the fourth ventricle communicates with the subarachnoid space. The 

 fourth ventricle, as it becomes continuous with the central canal of the spinal cord, 

 terminates in a point, the calamus scriptorius. From the inferior surface, the 

 cerebellar hemispheres are more definitely demarcated, and between them is the 

 vermis or central lobe of the cerebellum. 



Divisions of the encephalon. — The encephalon as a whole is developed from a 

 series of expansions, flexures, and thickenings of the wall of the cephalic portion of 

 the primitive neural tube, the three primary brain vesicles. Being continuous 

 with the spinal cord, it is arbitrarily considered as beginning just below the 

 level of the decussation of the pyramids, or at a line drawn transversely between 

 the decussation of the pyramids and the level of the first pair of cervical nerves. 



In its general conformation four natural divisions of the brain are apparent: 

 the two most enlarged portions — (1) the cerebral hemispheres and (2) the cere- 

 bellum; (3) the mid-brain (mesencephalon) between the cerebral hemispheres and 

 the cerebellum, and (4) the medulla oblongata, the portion below the pons and 

 above the spinal cord (fig. 602). However, the most logical and advantageous 

 arrangement of the divisions and subdivisions of the encephalon is on the basis of 

 their development from the walls of the embryonic brain vesicles. (See fig. 598.) 

 On this basis, for example, both the medulla oblongata and the cerebellum 

 with its pons arc derived from the posterior of the primary vesicles, and are, 

 therefore, included in a single gross division of the encephalon, viz., the rhomben- 

 cephalon. In the following outline the anatomical components of the enceph- 

 alon are arranged with reference to the three primary vesicles from the walls 

 of which they are derived, and the primary flexures and thickenings of the walls 

 of which they are elaborations. 



During the early growth of the neural tube its basal or ventral portion and the lateral por- 

 tions acquire a greater thickness than the roof of tlie tube, and thus the tube is longitudinally 

 divided into a basal or ventral zone and an alar or dorsal zone. This is especially marked in the 

 brain vesicles. iStruclures arising from the dorsal zone begin as localised thickenings of the 

 roof. For example, in the rliomhencophalon the greater part of the medulla oblongata and of 

 the pons region is derived from the ventral zone, wliile the cerebellum is derived from the dorsal 

 zone. The first of the flexures occurs in the region of the future mesencephalon, and is known 

 as the cephalic flexure; next occurs the cervical flexure, at the junction with the spinal cord; 



