776 



THE NERVOUS SYSTEM 



THE BRAIN OR ENCEPHALON 



The brain is that greatly modified and enlarged portion of the central nervous 

 system which is enclosed within the cranial cavity. It is surrounded and supported 

 by the same three membranes (meninges) that envelope the spinal cord. While there 

 is a considerable subdural space, the brain more nearly fills its cavity than does the 

 spinal co.-d. Exclusive of its dura mater, it weighs from 1100 to 1700 gm. (40-60 

 oz.), varying in weight with the stature of the individual or with the bulk of the tis- 

 sues to be innervated. Its average weight is 1360 gm. (48 oz.) in males and 1250 gm. 

 (44 oz.) in females. It averages about fifty times heavier than the spinal cord, or 

 about 98 per cent, of the entire cerebro-spinal axis. In its precocious growth it is at 

 birth relatively much larger than at maturity. At birth it comprises about 12 per 

 cent, of the total body-weight, while at maturity it averages only about 2 per cent, 

 of the weight of the body.' Its proportion to the body- weight averages somewhat 

 higher in smaller individuals. Its specific gravity averages 1.036. 



FIG. 575. MESIAL SECTION OF THE HEAD OF A FEMALE THIRTY-FIVE YEARS OLD. 



SUPERIOR SAGITTAL 



SIXUS 

 CORPUS CALLOSl/JU 



SEPTT7.V 

 PELLUCinrX 



THALAHUS 



Vein of Galen 



EPIPHTSIS 



Posterior cerebral 

 artery 



CORPORA 

 QUADRIOEXIXA 



THIRD NERVE 



STRAIGHT 

 SINUS 



CEREBELLUM 

 OCCIPITAL 



SIHUS 



FOURTH VENTRICLE 



sn.rrs cm- 

 G ULI 



FOR A. WE* OF 

 1IOXSO 



CRISTA K A I.I.I 

 '.rtbral 

 artery 



OPTIC CIIIA.1.VA 



SrilFSrtlDAL 

 SIXUS 



HYPOPHYSIS 

 \ 



.VElit-LI.A OB- 

 LOXGA TA 



The differences between the meninges of the brain and those of the spinal cord occur chiefly 

 in the dura mater. (1) The dura mater is about double the thickness of that of the spinal cord, 

 and, consists of two closely adhering layers, the outermost of which serves as the internal peri- 

 osteum of the cranial bones. (2) The inner layer is duplicated in places into strong partitions 

 which extend between the great natural divisions of the encephalon. Of these, the sickle-shaped 

 falx cerebri extends between the hemispheres of the cerebrum, the crescentic tentorium cerebelli 

 extends between the cerebellum and the overlapping posterior portion of the cerebrum, and the 

 smaller falx cerebelli occupies the notch between the hemispheres of the cerebellum. Contained 

 within these partitions of the dura mater are the great collecting venous sinuses of the brain. 

 These will be considered in the more detailed description of the cranial meninges. 



General topography. In its dorsal aspect or convex surface the encephalon 

 is oval in contour, with its frontal pole usually narrower than its occipital pole. 

 Viewed from above, the cerebrum comprises almost the entire dorsal aspect, the 

 occipital lobes overlapping the cerebellum to such an extent that only the lateral 

 and lower margins of the cerebellar hemispheres are visible. The great longitu- 

 dinal fissure of the cerebrum, separating the cerebral hemispheres, begins on the 

 ventral surface of the occipital lobes and extends anteriorly and ventrally between 

 the frontal lobes, so as to be evident on the base of the brain. 



