200 COMPARATIVE ANATOMY 



myelencephalon 1 (primary hind-brain). The telencephalon usu- 

 ally gives rise to a pair of lobes, the cerebral hemispheres, and the 

 mid-brain to a pair of optic lobes or corpora Mgemina dorsally, and to 

 two longitudinal bands, the crura cerelri, ventrally. The meten- 

 cephalon is also spoken of as the cerebellum, and the myelencephalon 

 as the ~bulb or medulla ollongata. From the secondary fore-brain 

 paired olfactory lobes 2 are given off anteriorly, and its floor or basal 



Olf Tff Z ZHKCJJKS'fl SJL WT 



' 



la. 



FIG. 149. LONGITUDINAL SECTION THROUGH THE SKULL AND BRAIN OF AN 

 (IDEAL) VERTEBRATE EMBRYO. (In part after Huxley.) 



Be, basis cranii ; Cc, central canal of spinal cord ; Ch, notochord ; HC, posterior 

 commissure ; HH, cerebellum (metencephalon, secondary hind-brain) ; MR, 

 mid-brain (mesencephalon) ; NH, primary hind-brain (myelencephalon) ; 

 NH 1 , nasal cavity ; SD, roof of skull ; VH, secondary fore-brain (telencepha- 

 lon), showing the corpus striatum (Cs) at the base, and the olfactory lobe 

 (OlJ] anteriorly; ZH, diencephalon (primary fore-brain), which has* given 

 rise dorsally to the pineal body (epiphysis, Z), and ventrally to the infundi- 

 bulum (7), to which the pituitary body (hypophysis, H) is attached : anteriorly 

 to this is seen the optic nerve (Opt), arising from the optic thalamus (Tho). 



portion becomes thickened to form a large " basal ganglion,' 5 the 

 corpus striatum, while its peripheral part is distinguished as the 

 "mantle " or pallium (Fig 149). 3 



The pallial region undergoes an important process of develop- 

 ment in passing upwards from lower to higher forms, gradually 

 becoming differentiated histologically into a layer of cortical gray 

 matter of great physiological importance, the relative differentiation 

 of which stands in close relation to the mental development of 

 the animal. The telencephalon reaches its greatest perfection in 

 Mammals, more especially in Man, while in certain lower Vertebrates 

 the cortex is partially or entirely non-nervous and retains its 



1 There is still a want of unanimity in the use of this nomenclature, and 

 many of the terms given above, as well as others with a similar ending, are used 

 in different senses by different morphologists. 



2 The olfactory lobes are of great importance phylogenetically, as the origin 

 of the telencephalon, and more especially of its deeper basal portion, is closely con- 

 nected with the olfactory organ. 



3 From the primarily epithelial pallium arises a median dorsal outgrowth, 

 the paraphysi* (Fig. 150), just anteriorly to a transverse fold of the epithelial 

 roof (velum transrersnm) which separates the ventricles of the primary and 

 secondary fore-brain, and on the edge of which a posterior pallial commissure can 

 be recognised (e.y. in Lizards) : this fold encloses a vascular thickening of the 

 pia mater (choroid plexus), from which the paraphysisis not always distinguishable 

 (cf. Fig. 165). The latter apparently represents a glandular organ, recalling that 

 connected with the infundibulum : whether it also includes the vestige of a sensory 

 apparatus, like the parietal and pineal organs, is doubtful (cf. pp. 202 and 20.3). 



