THE BRAIN 153 



As regards its minute structure, two parts can be distinguished 

 in the spinal cord, & white substance, consisting of nerve-fibres only, 

 and a gray substance, composed of nerve-cells as well as fibres. Their 

 relative positions vary in the different animal groups, as well as in 

 the different regions of the cord ; the white substance, however, 

 has typically a more peripheral, the gray a more central position, 

 the latter surrounding the central canal and usually presenting a 

 pair of dorsal and ventral cornua in transverse section. 



2. The Brain. 



Before the medullary groove becomes closed, the anterior ex- 

 panded part of the medullary tube presents three swellings, which 

 are spoken of as the primary fore-, mid-, and hind-brain, or anterior, 

 middle, and posterior cerebral-vesicles (Fig. 124) ; the cavities of 

 the vesicles (ventricles) are in direct connection with the central 

 canal of the spinal cord. Botli the primary fore- brain and hind- 

 brain then become differentiated, each into two parts, and thus five 



FIG. 124. DIAGRAM OF THE EMBRYONIC CONDITION OF THE CENTRAL NERVOUS 



SYSTEM. 



G, brain, with its three primary vesicles, 7, //, /// ; R, spinal cord. 



divisions of the brain may be distinguished. Counted from before 

 backwards these are : prosencephalon (secondary fore-brain), thala- 

 mencephalon (primary fore-brain), mesencephalon (mid-brain), meten- 

 cephalon (secondary hind-brain), and myelencephalon (primary 

 hind-brain). The prosencephalon usually gives rise to a pair of 

 lobes, the cerebral hemispheres, and in the mid-brain a pair of 

 optic lobes or corpora bigemina become differentiated dorsally, and 

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

 cephalon is usually spoken of as the cerebellum, and the myelen- 

 cephalon as the medulla oblongata. 



From the base of the prosencephalon or hemispheres paired 

 olfactory lobes (rhinencephala} are given off anteriorly, and the floor 

 or central part of each hemisphere becomes thickened to form a 

 large " basal ganglion," the corpus striatum, while its peripheral 

 part is distinguished as the " mantle" or pallium (Fig. 125). 



The relative development and differentiation of the pallium 

 stands in close relation to the mental development of the animal, and 

 reaches its greatest perfection in Mammals, especially in Man. In 

 certain Fishes the pallium remains partially or entirely non- 

 nervous, retaining its primitive epithelial character, and a layer 



