672 THE NERVOUS SYSTEM. 



other by an S-shaped vertical lamina of white matter, called the internal medullary 

 lamina. This is named internal in contradistinction to a second or external medullary 

 lamina of white matter, which coats the outer surface of the optic thala-nms arid 

 connects it with the internal capsule. The inner nucleus is connected with the 

 corresponding nucleus of the opposite side through the middle commissure of the 

 third ventricle. The external nucleus, which is the larger of the two, extends 

 backward into the pulvinar. The gray matter of the optic thalamus contains 

 large multipolar and fusiform cells, and is traversed in every direction by numerous 

 nerve-fibres. 



The optic thalamus is intimately connected with the following structures : 



1. It constitutes a relay for the greater number of the fibres of the tegmentum 

 of the crus cerebri. 



2. The pulvinar receives many of the fibres of the optic tract. 



3. It is connected with the cerebral cortex, (a) through the anterior stalk of 

 the optic thalamus, which passes from the anterior extremity of the thalamus 

 through the anterior limb of the internal capsule to the frontal lobe ; (6) through 

 the posterior stalk or optic radiations, consisting of fibres which take their origin 

 in the pulvinar and are transmitted through the extreme posterior part of the 

 internal capsule to the occipital lobe ; (c) through the inferior stalk or ansa 

 peduncularis, made up of fibres which leave the inferior surface of" the thalamus 

 and end in the temporal lobe; (d) through fibres which pass from the external 

 surface of the thalamus to the parietal lobe. 



4. With the corpus striatum. The fibres destined for the caudate nucleus leave 

 the external surface ; those for the lenticular nucleus, the inferior aspect of the 

 thalamus. 



5. With the corpus albicans through the bundle of Vicq d'Azyr. 



In connection with the optic thalamus two small nuclei of gray matter require 

 consideration : (1) One of these, the anterior nucleus, is situated in the anterior 

 tubercle of the optic thalamus. This nucleus receives the fibres (bundle of Vicq 

 d'Azyr) which take origin in the cells of the corpus albicans (see page 657). 

 Though this bundle of fibres appears to be the direct continuation of the anterior 

 pillar of the fornix through the corpus albicans to the optic thalamus, it is be- 

 lieved to have no histological continuity with it. The fibres of the anterior 

 pillar of the fornix form synapses in the corpus albicans around the cells which 

 give origin to the bundle of Vicq d'Azyr, and thus an indirect communication 

 only is established between the fornix and the optic thalamus. (2) The second 

 gray nucleus lies in a depressed space, the trigonum habenula?, situated between the 

 pulvinar and the posterior part of the peduncle of the pineal gland. It is termed 

 the ganglion of the habenula. It receives fibres from the peduncles of the pineal 

 body, and sends off others which pass to a small collection of gray matter, 

 situated between the diverging crura cerebri, and named the ganglion inter- 

 pedunculare. 



The pineal gland (epiphysis cerebri}, so named from its peculiar shape (pinus, 

 a fir-cone), is a small reddish gray body, conical in shape (hence its synonym, 

 conariuni), placed immediately above and behind the posterior commissure and 

 between the anterior corpora quadrigemina, on which it rests. It is covered by the 

 velum interpositum, which intervenes between it and the splenium of the corpus 

 callosum. It is an upgrowth from the second cerebral vesicle (hence the name 

 epiphysis), and is at first hollow, but soon becomes solid and loses its connection 

 with the ventricular cavity. It is retained in its position by a duplicature of pia 

 mater, derived from the under surface of the velum interpositum, which almost 

 completely invests it. The pineal gland is about four lines in length and from 

 two to three in width at its base, and is said to be larger in the child than in the 

 adult, and in the female than in the male. It is attached on either side by a 

 flattened stalk of white matter, the pedunculus conarii. This stalk consists of two 

 laminae, upper and lower, separated by a little recess, the pineal recess (see page 

 671). The lower lamina is prolonged into the posterior commissure. The upper 



