706 THE NERVOUS SYSTEM. 



cerebellum the membrane is more delicate, and the vessels from its inner surface 

 are shorter. Upon the crura cerebri and pons Varolii its characters are altogether 

 changed ; it here presents a dense fibrous structure, marked only by slight traces 

 of vascularity. 



According to Fohmann and Arnold, this membrane contains numerous 

 lymphatic vessels. Its nerves are derived from the sympathetic, and also from 

 the third, fifth, sixth, facial, glosso-pharyngeal, pneumogastric, and spinal acces- 

 sory. They accompany the branches of the arteries. 



THE BRAIN. 



[By BEEN B. GALLAXJDET. M. D., 



Demonstrator of Anatomv, College of Physicians and Surgeons (Columbia University) -, Surgeon to Bellevue 



Hospital, New York City.] 



GENERAL CONSIDERATIONS AND DIVISIONS. 



THE Brain, or encephalon, is that portion of the cerebro-spinal axis which is 

 contained in the cranial cavity. It may be divided into five portions, which, from 

 below upward, are as follows : 1. The medulla oblongata ; 2. The pons Varolii 

 and cerebellum ; 3. The mid-brain ; 4. The inter-brain ; 5. The two hemispheres. 

 The inter-brain and the two hemispheres are sometimes grouped together as the 

 cerebrum. Commonly, however, the word "cerebrum" means the two hemi- 

 spheres only. 



These various subdivisions of the brain are based on the method of development 

 of the brain, each of which corresponds to one of the five cerebral vesicles into 

 which the original foetal brain, a mere tube, is soon divided. 



Authorities differ as to the precise method of development of the early foetal 

 brain after it has become a closed tube. Some observers state that this brain- 

 tube becomes partially constricted in two places, thus giving rise to three primary 

 cerebral vesicles, and that no further constrictions as such occur. Others claim 

 that, while this is true, soon afterward the anterior and posterior vesicles are fur- 

 ther subdivided by similar, though not so well-marked, constrictions. 



This latter method seems, perhaps, the simpler, and is the one which will be 

 followed in the present description. 



There are thus formed, first, three primary cerebral vesicles, and then five 

 secondary cerebral vesicles. The three former are known respectively as the 

 fore-brain, the mid-brain, and the hind-brain. Of the five secondary vesicles, the 

 first and second result from constriction of the fore-brain ; the third is the orig- 

 inal mid-brain unchanged, Avhile the fourth and fifth are derived from the hind- 

 brain in a manner similar to that in which the first two are formed from the fore- 

 brain. The first secondary vesicle is known as the prosencephalon ; the second, 

 as the thalamencephalon ; the third, as the mesencephalon, or mid-brain; the fourth, 

 as the epencephalon ; and the fifth, as the metencephalon (Fig. 412). Each of 

 these subdivisions, of course, contains its own portion of the original brain- 

 cavity, and these various portions are all in direct continuity, one with the 

 other. 



In comparing these divisions of the embryonal brain with those of the adult 

 brain already mentioned, it is found that the prosencephalon, together with the 

 thalamencephalon, develop into or go to form the inter-brain, and hence their 

 cavities make up the third ventricle, which is the name given to that portion of 

 the general brain-cavity, in the adult, included in the inter-brain. It is common, 

 however, in describing the adult brain to use the names " inter-brain " and 

 "thalamencephalon " interchangeably, thus disregarding the prosencephalon. The 

 reason for this is that the latter, in the adult brain, is merely the extreme ante- 

 rior part of the true thalamencephalon, while its cavity holds a similar relation 

 to the third ventricle i. e. it is only the anterior end of the ventricle. It is 

 that portion of the third ventricle which has on each side the opening known as 

 the foramen of Monro, the significance of which will be dwelt upon later. 



