904 



THE NER VE SYSTEM 



now in vogue tends to mislead. The relations of "diencephalon" and "telen- 

 cephalon" are further complicated by the intimate fusion of the sides of the former 

 (thalami) with the floors of the latter; this caudatothalamic fusion, in the adult 

 brain, gives rise to some difficulty in distinguishing the two segments. The 

 internal capsule which intervenes between thalamus and lenticular nucleus also 

 intervenes between lenticular nucleus and caudate nucleus, both telencephalic 

 parts. 



External Morphology. The diencephalon or thalamencephalon comprises the 

 thalami, the pineal body or epiphysis and habenulae, the external geniculate bodies, 

 and the pars mammillaris hypothalami. (Other classifications include also the 

 pars optica, with tuber cinereum, infundibulum, optic chiasm, and pituitary body or 

 hypophysis. It is also defined as so much of the fore-brain as does not enter into 



FORAMEN OF MONRO 



MIDDLE COMMISSURE 



CHOROID PLEXUS OF 

 THIRD VENTRICLE 



TAENIA THALAMI 



HABENULAR 

 COMMISSURE 



POSTERIOR 

 COMMISSURE 



ROSTRUM 



COPULA 



ANTERIOR 

 COMMISSURE 

 LAMINA TERMINALIS 



OPTIC CHIASM 



OPTIC 



NERVE 



PITUITARY BO 



AQUEDUCT 



SUP. MEDULLARY 



VELUM 

 FOURTH 

 VENTRICLE 



FIG. 664. Mesal aspect of a brain sectioned in the median sagittal plane. 



the formation of the cerebral hemispheres.) Caudad it is continuous with the 

 mid-brain, cephalad with the cerebral hemispheres. Its primitive cavity becomes 

 metamorphosed in the adult into the third ventricle or diacele as the lateral walls 

 hypertrophy to form the thalami. Its ventral surface is the relatively insignificant 

 gray lamina in the intercrural space. Its dorsal surface is concealed from view 

 by the massive hemispheres and their great commissure, the corpus callosum, 

 and by the fornix. Its actual roof, separating it from the overlapping cerebral 

 parts, is a delicate membranous fold, the velum interpositum or diatela. 



The Thalami. 1 The thalami constitute the bulk of this portion of the brain. 

 They are large ovoid masses of gray substance so named by the ancients after their 

 resemblance to a pair of couches. Each thalamus is smaller frontad than caudad 

 and the caudal ends are more widely separated from each other. The mesal or 



1 Thalamos, bed or couch; bed-chamber. 



