HEMISPHERES OF THE BRAIN. 643 



substance. At the base of the brain, in the situation of the anterior and posterior 

 perforated spaces, a number of long straight vessels are given off, which pass 

 through the white matter to reach the gray substance in the interior. On the 

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

 are shorter. The pia mater of the spinal cord is thicker, firmer, and less vascular 

 than that of the brain, and as it is traced upward over the medulla it is seen to 

 preserve these characters. At the upper border of the medulla it is prolonged 

 over the lower half of the fourth ventricle, forming a covering for it (tela choroidea 

 inferior) before it is reflected on to the under surface of the cerebellum. 



According to Fohmann and Arnold, this membrane contains numerous lym- 

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

 third, fifth, sixth, facial, glosso-pharyngeal, pneumogastric, and spinal accessory. 

 They accompany the branches of the arteries. 



THE BRAIN. 



GENERAL CONSIDERATIONS AND DIVISIONS. 



The encephalon or brain is that portion of the cerebro-spinal axis which is con- 

 tained in the cavity of the cranium. For purposes of description it may be 

 divided into five parts, as follows: (1) the two cerebral hemispheres; (2) the 

 inter-brain ; (3) the mid-brain ; (4) the pons Varolii and cerebellum ; and (5) the 

 medulla oblongata. If the student will refer to the section on the Development of 

 the Brain he will find that these five portions correspond fairly accurately to the 

 five secondary cerebral vesicles, of which the brain at an early period of embry- 

 onal life consisted : the prosencephalon, or Jirst vesicle, by means of a protrusion 

 from its front part on either side, forms the cerebral hemispheres and the lateral 

 ventricles ; the remainder of the prosencephalon, together with the second vesicle, 

 the thalamencephalon, form the inter-brain and Jffltird ventricle ; the third vesicle, 

 the mesencephalon, forms the mid-brain, or that portion which connects the inter- 

 brain and hemispheres above with the pons Varolii below, and the cavity of the 

 vesicle forms the aqueduct of Sylvius, or iter a tertio ad quartum ventriculum ; the 

 fourth vesicle, the epencephalon, becomes the future pons Varolii and cerebellum, 

 and its cavity forms the upper half of the fourth ventricle; and, finally, the fifth 

 Vesicle, the melencephalon, develops into the medulla oblongata, and its cavity 

 forms the lower half of the fourth ventricle. It will thus be seen that the five 

 divisions of the encephalon mentioned above correspond to the five secondary cere- 

 bral vesicles, with the exception of the first two, which together form the cerebral 

 hemispheres and the inter-brain. In consequence of this these two portions of 

 the brain are sometimes grouped together as the cerebrum. 



I. The Hemispheres of the Brain. 



General Considerations. The two hemispheres constitute the largest portion of 

 the encephalon, and, together with the parts derived from the thalamencephalon, 

 form what is called by some writers the fore-brain. They occupy the whole of the 

 vault of the skull, and consist of a central cavity, in either hemisphere, surrounded 

 by exceedingly thick and convoluted walls of nervous tissue. The under surface 

 or base of the cerebrum is of an irregular form, resting in front on the anterior 

 and middle fossae of the skull and behind upon the tentoriuni cerebelli. The 

 upper surface is of an ovoid form, broader behind than in front, convex in general 

 outline, and divided into two lateral halves or hemispheres, right and left, by the 

 great longitudinal fissure, which extends throughout the entire length of the cere- 

 brum in the middle line, reaching down to the base of the brain in front and 

 behind, but interrupted in the middle by a broad transverse commissure of white 

 matter, the corpus callosum, which connects the two hemispheres together. 



The Surface of the Cerebrum. Each hemisphere presents an outer convex sur- 

 face, filling the concavity of the corresponding half of the vault of the cranium ; 



