THE NERVOUS SYSTEM. 



125 



considerable distance, and finally terminate in ovoid corpuscles (Fig. 336) 

 closely resembling, and homologous with, the corpuscles in which the ulti- 

 mate ramifications of the branched cells of Purkinje in the cerebellum 

 terminate. Thus it would seem that the large pyramidal cells of the third 

 layer are themselves homologous with the cells of Purkinje in the cere- 

 bellum. 



The white matter of the brain, as of the spinal cord, consists of bundles 

 of medullated, and, in the neighborhood of the grey matter, of non- 

 medullated nerve-fibres, which, however, as is 

 the case in the central nervous system generally, 

 have no external nucleated nerve-sheath, which 

 are held together by delicate connective tissue. 

 The size of the fibres of the brain is usually less 

 than that of the fibres of the. spinal cord: the 

 average diameter of the former being about 

 TO <hnr of an inch. 



Chemical Composition. The chemistry 

 of ner re and nerve cells has been chiefly studied 

 in the brain and spinal cord. Nerve matter 

 contains several albuminous and fatty bodies 

 (cerebrin, lecithin, and some others), also fatty 

 matter which can be extracted by ether (in- 

 cluding cholesterin) and various salts, especially 

 Potassium and Magnesium phosphates, which 

 exist in larger quantity than those of Sodium 

 and Calcium. Yolk of egg resembles cerebral 

 substance very closely in its chemical composi- 

 tion; milk and muscle also come very near it. 



The great relative and absolute size of the 

 Cerebral hemispheres in the adult man, masks 

 to a great extent the real arrangement of the 

 several parts of the brain, which is illustrated in 

 the two accompanying diagrams. From these 

 it is apparent that the parts of the brain are 

 disposed in a linear series, as follows (from be- 

 fore backward) : olfactory lobes, cerebral hemis- 

 pheres, optic thalami, and third ventricle, cor- 

 pora' quadrigemina, or optic lobes, cerebellum, 

 medulla oblongata. 



This linear arrangement of parts actually 

 occurs in the human foetus (see Chapter on Development), and it is 

 permanent in some of the lower Vertebrata, e.g., Fishes, in which the 

 cerebral hemispheres are represented by a pair of ganglia intervening be- 

 tween the olfactory and the optic lobes, and considerably smaller than the 

 latter. In Amphibia the cerebral lobes 'are further developed, and are 

 larger than any of the other ganglia. 



FIG. 338. Diagrammatic hori- 

 zontal section of a Vertebrate 

 brain. The figures serve both 

 for this arid the next diagram. 

 Mb, mid brain: what lies in front 

 of this is the fore, and what lies 

 behind, the hind brain; Li, la- 

 mina terminalis; Olf, olfactory 

 lobes; Hmp, hemispheres: Th. 

 E, thalamencephalon ; Pa, pineal 

 gland; Py, pituitary body: I-'.M. 

 foramen of Munro; cs, corpus 

 striatum; Th, optic thalamus; 

 CC, crura cerebri: the mass lying 

 above the canal represents the 

 corpora quadrigemina; Cb, cere- 

 bellum; I IX., the nine pairs of 

 cranial nerves ; 1 , olfactory ven- 

 tricle; 2, lateral ventricle ; 3. third 

 ventricle ; 4, fourth ventricle : + , 

 iter a tertio ad quartum ventri- 

 culum. (Huxley.) 



