'////; ENCEPHALON 779 



emerge from I lie 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 outwards in company with the vagus. The root 

 filaments of the vagus and glosso-pharyngeal 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. 



The hypoglossal or the twelfth and last of the cranial nerves emerges as a series 

 of rootlets between the pyramid and the olive. Thus it arises nearer the mid-line, 

 and in line with 1 he abducens, trochlear, and oculomotor. 



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

 tcntorium cerebelli removed, the corpora quadrigemina (mesencephalon) may be 

 observed. These are situated above the cerebral peduncles, in the region of the ven- 

 tral appearance of the oculomotor and trochlear nerves. Nesting upon the supe- 

 rior pair of the quadrigemiiiate 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). 



My 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-medullaris, a subarachnoid space in this region, 

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

 tela chorioidea (ligula) and plexus chorioidea 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, ter- 

 minates in a point, the calamus scriptorius. From the inferior surface the cere- 

 bellar 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 anterior portion of 

 the primitive neural tube. Meing 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 cerebellum; 

 (3) the corpora quadrigemina (mesencephalon) between the cerebral hemispheres and 

 the cerebellum, and (4) the medulla oblongata. the portion continuous with the spinal 

 cord. 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. On this basis, for example, both the medulla oblon- 

 gata and the cerebellum with its pons are derived from the posterior of the primary 

 vesicles, and are. therefore, included in a single gross division of the encephalon, viz., 

 the rhomhencephalon. In the following outline the anatomical components of the 

 encephalon are arranged with reference to-lhe 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 acquires 

 a greater thickness than the lateral portions and the roof of the 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. Structures arising from the 

 dorsal zone beirin as localised thickening of the roof. For example, in the rhomb- 

 encephalon the greater part of the medulla oblongata and of the pons region is 

 derived from the ventral /.one. while the cerebellum is derived from the dorsal /one. 

 The lir-t 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: third, the pontine flexure, in the region of the future fourth 

 ventricle. Both the cervical and pontine flexures, while having a significance in the 

 growth processes, are almost entirely obliterated in the later growth of the 

 encephalon. 



