CHAP. IX.] NERVOUS SYSTEM AND ORGANS OF SENSE. 269 



lateral f/>/nis (Figs. 125 and 126, .s) rims stralglit from behind for- 

 wards till it comes to a transverse furrow, the crucial sulcus (<?), 

 round wliicli it bends, running inwards again, and finally turns 

 outwards and downwards round another sulcus called the supra- 

 orbilal (o). The next gyrus is the middle lateral (m), which curves 

 outwards and downwards at each end. Finally comes the inferior 

 lateral <Ji/rus (/), which is seen, in this view, to be somewhat divided 

 toAvards each end by the terminations of two ascending sulci. 



AVhen the brain is vertically bisected, we see, above, the superior 

 lateral gyrus traversed by a feebly indicated antero-posterior groove, 

 while a deep sulcus, the ealloso-marr/inal sulcus, divides it incom- 

 pletely from the hip2)0campal gurus (Fig. 129, h), which immediately 

 adjoins the corpus callosum, behind which it dips down, and passing 

 round the crus cerebri of that side runs forwards the marked promi- 

 nence of the temporal lobe (Fig. 128, h). The sulcus on the poncave 

 side of the lower part of this gyrus forms (by projecting into the 

 descending cornu of the lateral hemisphere) the /lipjjocamjms major, 

 which circumstance gives its name to this gyrus. At the anterior 

 end we see the crucial sulcus (Fig. 129, c), with the internal surface 

 of the superior lateral gyrus beneath it. 



When the brain is seen in profile, we see at its anterior end the 

 sharp bend upwards and downwards of the superior lateral gyrus 

 around the supra-orbital sulcus (Fig. 125, o) and then its ascent 

 behind the crucial sulcus (c) ; after which it runs back along the 

 summit of the hemisphere. Within this is the median lateral fold (;«), 

 while the whole space embraced by it is occupied by the inferior 

 lateral gyrus traversed by two ascending sulci, the summits of which 

 sulci appear at the side of the upper view of the brain. Next to 

 be noted is the small but very important Sylvian fissure (s), which 

 forms as it were the axis round which all these convolutions are 

 disposed. Finally the anterior end of the hippocampal gyrus (//) 

 makes its appearance beneath and in front of the Sylvian fissure. 



When the under surface of the brain is in view we see the lower 

 anterior ends of the three lateral gyri, and the large expanded 

 termination of the hippocampal gyrus is the temporal lobe. 



§ 8. Having now reviewed the nervous centres or cerebro-spinal 

 axis, we may proceed to consider the peripheral part of the 

 nervous system, i.e., the nerves which are given off by the axial part 

 of that system. 



The nerves which go forth from the cerebro-spinal axis to different 

 parts of the body are bilaterally symmetrical, there being (beyond 

 the olfactory nerves) a pair (one right and one left) of each. 



The most anteriorly-situated nerves attain their destination after 

 passing through the foramina of the skull, on wliich account they 

 are denominated cranial nerves. 



Of these there are twelve, as follows : — 



1. Olfactory. 



2. Optic. 



3. Oculo-motor. 



