THE BRAIX AXD THE WALL,^ OE THE CRAMAL rAVETY 727 



The adult male brain weiKlis on an average 49 '5 ounces; the female brain is somewhat 

 lighter, and weighs from 5 to 5 '5 ounces less than the male. 



The cerebrum forms about 87'5 i)er cent, of the entire brain ; the cereVjellum about 10'5 per 

 cent. ; and the pons and medulla about 2 per cent. 



Weight (if the Spinal Cord 



In the adult, the spinal cord, when all extraneous structures have been removed, weighs from 

 1 to 1 5 ounce-s. 



For description of C'ranio-eerebral Topography see Section X. 



THE RELATIONS OF THE BRAIX TO THE WALLS OF THE 

 CRAXIAL CAVITY 



In this section the suliject of cranio-cerebral topography will be dealt with from 

 a purely anatomical standpoint, for the precise methods by which the exact posi- 

 tions of the more important fissures, sulci, convolutions, and areas can be ascer- 

 tained and mapped out on the surface of the head in the living subject are fully 

 (lescribed in Section X. whilst here only a very general survey of the relations of 

 the brain to the cranial bones will be given. 



The parts of the V)rain which lie in close relation Avith the walls of the cranial 

 cavity are the olfactory bulb and tract, the inferior and outer surfaces of the cerebral 

 hemispheres, the lower surfaces of the lateral lobes of the cerebellum, the anterior 

 surfaces of the medulla and pons, and the pituitary body. 



Certain of these portions of the brain lie in relation wdththe basi-cranial axis, — 

 that is, with the basi-occipital, the basi-sphenoid, and the ethmoid bones, — whilst 

 others are separated by the membranes only from the sides and vault of the cranial 

 space. Considering the former portions first, the anterior surface of the medulla 

 oblongata, which is formed by the anterior pyramids, lies, posteriorly, ujion the 

 u|)per surface of the basi-occipital bone. More anteriorly the anterior surface of 

 the pons rests upon the basi-sphenoid, from which it is partly separated by the 

 basilar artery and the sixth pair of cranial nerves. In front of the dorsum sellse 

 the pituitary body is lodged in the pituitary fossa; still further forwards the olfactory 

 tracts lie in grooves on the upper surface of the presphenoid section of the sphe- 

 noid bone; and in front of the sphenoid the olfactory bull)s rest upon the cribriform 

 plates of the ethmoid. 



Behind and laterally to the posterior part of the foramen magnum the lateral lobes 

 of the cerebellum are in relation with the cranial wall, resting upon the lower parts 

 of the supra- and the posterior parts of the ex-occipital i>ortions of the occipital 

 bone, whilst, anteriorly, each is in relation with the inner surface of the mastoid pro- 

 cess and the posterior surface of tlie petrous portion of the temporal bone. The 

 area of the skull wall which is in close relationship with the cerebellar hemispheres 

 may be indicated, on the external surface of the skull. l»y a line which commences 

 from the lower part of the external occipital protuberance; thence it runs upAvards 

 and outwards. It crosses the superior curved line a little beyond its centre, and, 

 continuing in the same direction, it crosses the lower part of the lambdoid suture 

 and reaches a point directly above the asterion (the meeting-] )oint of the occipital, 

 temporal, and j)arietal bones), Avhence it descends, just in front of the occipito- 

 mastoid suture, to the tip of the mastoid process; there it turns inwards to its ter- 

 mination at the margin of the foramen magnum, immediately behind tlie posterior 

 end of the occipital condyle. 



The other portions of the brain which lie in close relation with the cranial walls 

 are the lower and external surfaces of the cerebral hemis})heres. 



The lower surface of each cerebral hemisphere consists of two parts, an anterior 

 and a posterior, Avhich are separated by the stem of the Sylvian fissure. The 

 anterior part, formed bA'the orbital surface of the frontal lobe, rests upon the upper 



