THE DIENCEPHALON 665 



arated from the nucleus caudatus by an oblique groove in which there is a band 

 of white matter termed the stria terminalis or tsenia semicircularis. Internally it is 

 bounded by a narrow band, the stria medullaris, on which is a delicate ridge of 

 ependyma termed the taenia thalami. The strice unite posteriorly and blend with 

 the stalk of the pituitary body. Near this point they present a small enlargement 

 caused by the nucleus habenuhp. Anteriorly there is a small eminence, the anterior 

 tubercle. The posterior part of the thalamus has the form of a rounded ridge which 

 is continuous laterallj" with the optic tract/ Behind the point of origin of the 

 tract, in the angle b(>t\veen the thalamus and the cerebral peduncle, is the internal 

 geniculate body (Corpus geniculatum mediale), a well-defined oval prominence. 



The outer surface is separated from the lenticular nucleus by the internal capsule, an 

 important mass of white matter composed of fibers passing to and from the cerebral cortex. 

 These fibers fjo to form a larfje part of the ventral portion (basis) of the cere!)ral peduncle. From 

 the entire external surface of the thalamus fibers pass into the internal capsule and radiate to 

 reach the cerebral cortex; similarly fibers coming from the cortex converge in the internal capsule 

 to enter the thalamus. This arrangement is termed the thalamic racliation. Ventral to the 

 thalamus proper is the subthalamic tegmental region. This is the continuation of tlie tegmental 

 part of the cerebral pcduiich* into tlic dicnccphalon. It contains the red nucleus (Nucleus 

 ruber) an important ganglion on tiie course of tlie motor tracts. It receives numerous fibers 

 from the cerebral cortex and the corpus striatum. From it fibers proceed to the thalamus 

 and to the spinal cord; the filx-rs to the conl (Tractus rubro-.spinalis of Monakow) cross to the 

 opposite side and extend back in the tegmentum to the lateral columns of the cord. Lateral to 

 the red nucleus a conspicuous lenticular area of dark gray matter is vi.sil)le on cross-sections of 

 the subthalamic region; this is th(> subthalamic nucleus (Nucleus hypothalamicus s. corpus 

 Luysi), which consists of pigmented nerve-cells scattered through a dense network of fine medul- 

 lated fibers, and is richly supplied with capillary blood-vessels. The two nuclei are connected 

 by a transverse commissure (("ommi.ssura hypothalamica), which crosses the floor of the third 

 ventricle above the maminillary body. 



The pineal body or epiphysis is a small ovoid or fusiform red brown mass 

 situated in a deep central di^pression between the thalami and corpora quadrigemina. 

 It is variable in size, but is commonly about 10 to 12 mm. long and 6 mm. wide. 

 It is attached at the postero-superior quadrant of the third ventricle by a short 

 stalk, in which is a sm.all recess of that cavity. Its base blends in front with the 

 junction of the strise medullares of the thalamus. Immediately under the posterior 

 part of the stalk is a short transverse band of white matter, the posterior commis- 

 sure. 



The pineal body is inclosed in a fibrous capsule from which numerous trabeculse pass inward, 

 dividing the organ into spaces occupied by round epithelial cells of the same origin as the epen- 

 dyma of the ventricle. 



The mammillary body (Corpus mammillare) is a white round elevation a little 



larger than a pea which projects ventrally at the anterior end of the median furrow 



of the interpeduncular space. While it is a single body in external form in the horse, 



sections show that it is double in structure and contains a nucleus of gray matter 



on either side. 



Three sets of fibers are connected with the mammillary body. The anterior pillar of the 

 fornix curves down in the lateral wall of the third ventricle to the body and many of the fornix 

 fibers end in it. A bundle (Fasciculus thalamo-mammillaris) passes upward and backward 

 from it into the anterior part of the thalamus, and a tract (Fasciculus pedunculo-mamnnllaris) 

 extends back in the floor of the third ventricle to the tegmentum of the mid-brain. 



The pituitary body or hypophysis was mentioned as covering part of the inter- 

 peduncular space. It is oval in outline, flattened dorso-ventrally, and nearly an 

 inch (ca. 2 cm.) in width. It is attached by a delicate tubular stalk, the infundi- 

 bulum, to the tuber cinereum, a small gray prominence situated between the optic 

 chiasm in front and the mammillary body behind. 



The pituitary body consists of two parts which can be distinguished readily on sections by 

 their color. The glandular (or anterior) lobe is brown in color and is inclosed m a fibrous capsule. 



' This backward projection of the thalamus is equivalent to the pulvinar and lateral genicu- 

 late body of man, which are not superficially divided in the domestic animals. 



