238 SPECIAL PHYSIOLOGY. 



want of development of the cerebrum, the cerebellum is disproportion- 

 ately large, the ratio having been found to be about 1 to 4, or even as 

 low as 1 to 2.6. The spinal cord weighs usually, on an average, one 

 ounce and a half; its proportion to the encephalon is therefore about 

 1 to 33. The following numbers have been stated by Bourgery, to 

 represent the relative weights of the several parts of the nervous cen- 

 tres in man: the cerebrum, 170; the cerebellum, 21 ; and the pedun- 

 cles, and certain parts connected with them, named the corpora striata 

 ancl optic thalami, with the pons Varolii and the medulla oblongata, 

 13. The whole encephalon is thus supposed to be divided into 204 

 parts. 



The cerebrum. In considering further the structure of the great 

 nervous centres, it is important to bear in mind the primary fact, that 

 they are composed, cerebrum, cerebellum, pons, medulla oblongata, 

 and spinal cord, of two symmetrical halves, applied to, and united to- 

 gether, in various ways, along the middle line. Thus, the cerebrum 

 itself, Fig. 57, is composed of two lateral halves, called the cerebral 



Fig. 57. 



Fig. 57. Upper surface of the cerebrum of man. showing its subdivision, by the longitudinal fissure, into 

 two lateral hemispheres, right and left; also the chief suld and convolutions. The fissures of Rolando are 

 the oblique fissures which commence near the middle line, and proceed outwards and forwards, marking 

 off the frontal lobes, a. a, from the parietal lobes, which lie behind those fissures. The occipital lobes, 

 forming the hinder extremity of the hemispheres, completely conceal the cerebellum. 



hemispheres, a, a, which are united together, at the bottom of a deep 

 fissure, named the longitudinal fissure, by a thick transverse band of 

 nervous substance, called the corpus callosum, which, however, passes 

 across, between the hemispheres, in only a certain middle portion of 

 their extent, so that the latter, as shown on a horizontal section, Fig. 

 58, a, a, are completely separate, in front and behind it. It is in 

 this longitudinal fissure that the falx cerebri, Fig. 9, /, dips down; 

 whilst the tentorium is placed horizontally, between the hinder part 

 of the cerebral hemispheres and the upper surface of the cerebellum. 

 At the base of the cerebrum, its hemispheres are further connected by 

 their respective peduncles, which are themselves united together in the 

 middle line. Within the hemispheres, and between the corpus callosum 

 above and the diverging peduncles below, are certain cavities in the 



