VERTEBRA TJ:s. 



33 



the tii-st spiiml nerve ; ;', rhiiieiicepliMlon ; ji, prosence|ilitiloii 

 roof of thiihiineiicephiiUiu ; A, hypoiihysis ; m, iiieseiicephalMii, or oplic 

 lobes (corpora bigeniina) ; e, epeiicephaloii with thick roof, the ccrebcl- 

 luiii ; int, iiietencephalou, with thin roof (choroid plexus). 



thin roof of the tlialainoca'le is free on tbe upper surface ; behind tliat are the optic 

 lobes, or cor])ora bigeniina, of the mesenceplialon, succeeded by the comparatively 

 small cerebellum and the thin roof of the metaca'le. 



A greatly advanced stage of development is to be met with in the birds (Fig. 38), 

 the chief characteristics of (he brain of whicli consist in tlie great size and shape of the 

 cerebral hemisjiheres, which, 

 growing backwards towards 

 the cerebellum, cover the roof 

 of the thalaracncephalon, and 

 cause the optic lobes to as- 

 sume a lateral instead of a 

 dorsal ])Osition on the mesen- 

 cephalon. The cerebellum 

 itself grows forward to meet 

 the cerebral hemispheres, and 

 backwards so as to overlap the thin roof of the nietacoeles, and develops on either side 

 a small lateral lobe which becomes of far greater importance in the mammalian organ. 



So greatly are the cerebral hemispheres and the cerebellum developed in the main- 

 malian brain, that the other regions are entirely dwarfed in proportion. This is true 

 of even the lowest forms, but it reaches its maximum in man, where the cerebral 

 hemisjiheres even cover the cerebellum, as well as all the intervening parts of the brain. 

 Not only does this growth take place backwards, but also forwards, in front of the 

 lamina terminalis, and downwards towards the base of the brain, so that the lateral 

 ventricles, instead of being merely in front of 

 the aula, are continued by so-called 'cornua,' 

 into the lobes which result from the growth of 

 the cerebral hemispheres in the directions in- 

 dicated. 



In all of the lower forms the right and left 

 halves of the different regions of the brain are 

 connected by bundles of nerve fibres, constitut- 

 ing transverse commissures; so also longitudi- 

 nal commissures are present, connecting the 

 various regions of the brain witii each other. 

 The mammalian brain not only possesses representatives of these commissur.al systems, 

 but others are developed in response to the greater increase in importance of the 

 cerebrum and cerebellum, which reach their maximum in man. 



Of these we have already alluded to the pons Varolii, found on the ventral as]iect or 

 base of the brain, and serving to unite the lateral lobes of the cerebellum, which have 

 become in the lower forms of as great size as the original middle lobe, while in the higher 

 forms tliey greatly exceed it. Two other important commissures — the corpus callosum 

 and the fornix — are developed in connection with the cerebral hemispheres; but these, 

 instead of being within the coeliae, like the commissures of the lower forms, are really 

 outside this system of cavities. If a triangular ])atch of the median wall of each cere- 

 bral hemis]iherc be conceived to remain thin, and to unite with that of the other side, 

 instead of being uniformly thick and inde])endent from its neighbor, as in the turtle (Fig. 

 36), we shall realize the way in which the ' septum lucidum ' of the mammalian brain is 

 formed. Occasionally the union of the two halves of this thin septum is not complete, 



VOL. III. — 3 



i^'^'/' 



Fig. 38.— Brain of a gallinaceous bird ; c, cere- 

 brum ; c/, cerebellum ; m. medulla oblongata; 

 o,optic lobes; p, pituitary body; / — J'/,cranial 

 nerves. (The cut ends of fniul VII — X/ are 

 seen on tbe sides of the medulla.) 



