PARTS DERIVED FROM THE FORE-BRAIN 907 



cerebral cortex. As an "emotional" centre it is also under the inhibitory influence 

 of the cerebral cortex, which, if the emotion be not too strong, prevents its external 

 manifestation. 



The thalamocortical and corticothalamic fibres, with the internal capsule, enter 

 into the corona radiata or fan-like formation of the white substance of the cerebral 

 hemisphere. Although there is no anatomic subdivision into distinct groups of 

 these fibres as they stream to and from the thalamus, it is customary to distinguish 

 a frontal, a parietal, an occipital, and a ventral stalk. The frontal and parietal 

 stalks, as their names indicate, pass between the thalamus and frontoparietal 

 cortex, as well as to the lenticular nucleus and caudate nucleus. The occipital 

 stalk is composed of fibres passing in both directions between the pulvinar and 

 occipital cortex, constituting the so-called optic radiation. The ventral stalk 

 comprises the ansa lenticularis (thalamolenticular) and the ansa peduncularis 

 (thalamotemparal and thalamolnsular). They will be described in detail farther on. 



The external geniculate body, or pregeniculum, is an intercalar ganglion proper 

 to the optic nerve, derived from the thalamus. On section it is seen to be charac- 

 terized by the regular alternation of deeply gray and white laminae. The latter 

 are thicker and composed of fibres which enter the lateral geniculate body from 

 the optic tract and optic radiation. The nerve cells in the gray substance are 

 large, multipolar, and pigmented. 



[NOTE. The external geniculate body and the more isolated internal geniculate 

 body are generally included under the head of metathalamiis.] 



The hypothalamic tegmental substance, continuous with the mid-brain tegmen- 

 tuin, is interpolated between the ventral face of the thalamus, the red nucleus, 

 and a continuation of the substantia nigra known as the corpus hypothalamicus 

 or body of Luys. Through the hypothalamic tegmentum stream the fibres of 

 the medial lemniscus, of the superior peduncle of the cerebellum, and from the red 

 nucleus, to end in relation with thalamic cells. The corpus hypothalamicus is 

 a grayish-brown, lentiform mass which lies in the ideal continuation frontad of 

 the lateral part of the substantia nigra, and, like it, situated between pes and teg- 

 mentum. It is made up of fine myelinated fibres crowded in great profusion 

 and confusion, with numerous delicate, coiled capillaries and sparse, multipolar, 

 more or less pigmented, nerve elements of moderate size. The outline of the body 

 is defined by a white capsule, some of the fibres of which are seen to decussate 

 in the floor of the third ventricle with those of the opposite side, dorsocaudad of 

 the corpora albicantia. 



The Pineal Body (epiphysis; corpus pineale} (Figs. 664 and 665). The pineal 

 body (from its shape resembling a fir-cone pinus) is a small, reddish-gray 

 body placed between the caudad ends of the thalami and occupying the depression 

 between the two superior quadrigeminal bodies. It is covered by the velum inter- 

 positum, which intervenes between it and the splenium of the corpus callosum. 

 It is an outgrowth which is not regarded as an important neural ingredient of 

 the human brain and is generally believed to be a rudimentary relic, representing 

 a cyclopean eye 1 of some extinct ancestral vertebrate, homologous with the parietal 

 organ, resembling a molluscan eye of a living species of lizard (the Hatteria of 

 Australia). Its attached base is a hollow peduncle divisible into a dorsal and 

 ventral part by the intrusion of the epiphyseal recess (recessus pinealis) of the third 

 ventricle. The dorsal stalk continues on either side and upon each thalamus 

 "as the stria medullaris ; it is reenforced by commissural fibres joining the habenulse 

 of the two sides; hence another name for the dorsal stalk is the habenular com- 



1 Although most vertebrates show a single pineal body or parietal organ, it is double in the lamprey and cer- 

 tain reptiles; the two bodies lie one in front of the other not side by side (although probably paired organs 

 originally). The frontal organ sends its fibres into the habenular nucleus; the caudal organ to the region of 

 the posterior commissure (tectum opticum). 



