608 THE NEKVOUS SYSTEM. 



into two parts an anterior, termed the telencephalon, and a posterior, called th 

 diencephalon, which forms the greater part of the walls of the third ventricle. Th 

 extreme anterior part of the third ventricle belongs to the telencephalon, and thi 

 includes the anterior wall of the neural tube, which is known as the lamiu 

 terminalis. 



DEVELOPMENT OF PARTS DERIVED FROM FORE-BRAIN. 



The alar part of each side wall of the telencephalon is pushed out to form 

 diverticulum, which ultimately constitutes the cerebral hemisphere, and thus, from a vei 

 early period, the primitive position of this part of the side wall is indicated by tl 

 wide foramen interventriculare, or aperture of communication between the cavity of tl 

 cerebral hemisphere and the third ventricle (Fig. 538). 



The alar part of the side wall of the diencephalon is utilised for the developmei 

 of the thalamus, the epithalamus, and the metathalamus. Of these the thalamus 

 derived from the anterior and by far the greatest part of the alar wall. It arises as 

 large oval swelling, which gradually approaches its fellow of the opposite side, and thi 

 diminishes the width of the third ventricle. Finally, the two bodies sometimes come im 

 contact in the median plane and cohere over an area corresponding to the massa inte 

 media. This may occur about the end of the second month. 



From that section of the side wall to which the name of metathalamus is given tl 

 two geniculate bodies arise. Each of these shows, in the first place, as a depression c 

 the inside, and a slight elevation on the outside, of the wall of the diencephalon. As tl 

 thalamus grows backwards, it encroaches greatly upon the territory occupied by the ger 

 culate bodies. It thus comes about that in the adult brain the medial geniculate boc 

 seems to hold a position on the lateral aspect of the mesencephalon, whilst the later 

 geniculate body, viewed from the surface, appears to be a part of the thalamus. 



From the epithalamic region of the wall of the diencephalon are developed tl 

 pineal body, its peduncle, and the habenular region. These parts are relatively much mo: 

 evident in the embryonic than in the adult brain. The pineal body appears to 1 

 developed as a diverticulum of the posterior part of the roof of the diencephalon, but 

 reality it is a derivative of the alar lamina. Viewed from the dorsal aspect of the brai 

 tube, this diverticulum shows in the first instance, as a rounded elevation, from eac 

 side of which a broad ridge runs forwards. This ridge becomes the tsenia thalan 

 whilst in the region of its junction with the pineal elevation the trigonum habenul 

 takes shape. The pineal diverticulum ultimately becomes solid, but a small portion < 

 the original cavity is retained as the recessus pinealis of the third ventricle. 



The part of the diencephalon and telencephalon which represents the basal lamina (i. 

 lies below the level of the sulcus hypothalamicus) retains its primitive form, and undergo< 

 only slight change. Consequently, when this region in the adult brain is compared with tl 

 corresponding region in the embryonic brain, the resemblance between the two is verystrikinj 



In the fore-brain, therefore, it is the alar lamina which plays the predominant part i 

 the formation of the cerebrum. The value, also, of the basal part of the wall of th 

 portion of the neural tube is still further reduced by the fact that it no longer contaii 

 the nuclei of origin of efferent nerves. The highest of these nuclei (the oculomotor) 

 placed in the mesencephalon. [Johnston has recently announced the discovery of a sensor 

 nerve (nervus terminalis) attached to the fore-brain in human embryos ; and of course tl 

 optic and olfactory nerves enter the fore-brain.] 



The region of the fore-brain which lies below the sulcus hypothalamicus is termed tl 

 hypothalamus. The part of this, which corresponds to the diencephalon is called tl 

 pars mamillaris hypothalami, whilst the part in front, which belongs to the tele 

 cephalon, receives the name of pars optica hypothalami. From the pars mamillar 

 hypothalami are derived the corpus mamillare and a portion of the tuber cinerem 

 With the pars optica hypothalami are associated the following parts, viz., the tub' 

 cinereum, with the inf undibulum and the cerebral part of the hypophysis, the opt 

 chiasma, the optic recess, and the lamina terminalis. The corpora mamillaria fon 

 in the first instance, a relatively large ventral bulging of the floor of the brain-tut i 

 As development goes on this bulging becomes relatively small, and about the four 

 month the single projection becomes divided into the two tubercles. The infundibulu 

 and posterior or cerebral lobe of the hypophysis are developed as a hollow downwa 

 diverticulum of the floor of the telencephalon (Fig. 538). A portion of the origii 

 cavity is retained in the upper part of the infundibulum, and constitutes the infundibul 

 recess in the floor of the third ventricle. 



