116 ANTHROPOLOGY. 



The pineal fjland. This body, rendered famous by the vague theory of 

 Descartes, which viewed it as the chief source of nervous power, is placed 

 just behind the third ventricle, resting in a superficial groove which passes 

 along the median line between the corpora quadrigemina. It is of a grey- 

 ish color, heart-shaped, with the apex directed backwards and downwards. 

 A process from the deep layer of the velum interpositum envelopes it and 

 retains it in place. From each angle of the base there passes off a band of 

 white matter to the inner surface of each optic thalamus, called the pedun- 

 cles or hahence ol the pineal gland. The only connexion with the brain is 

 by means of these habenae. In the adult, grains of sandy matter are usually 

 found in the pineal gland, which are mostly collected in a cavity towards 

 its base ; sometimes, however, situated on the surface. 



The soft commissure is a lamina of light grey matter, situated in the 

 third ventricle, and extending between the optic thalami of opposite sides. 

 It forms a transverse horizontal plane dividing the ventricle into two 

 portions. 



We come next to consider that part of the brain which lies intermediate 

 to the cerebrum, cerebellum, and medulla oblongata, and conveniently 

 termed the mesocephalon. The pineal body rests upon its upper surface ; 

 but from its internal relation to the third ventricle and optic thalami, we 

 found its description with them most convenient. 



3. The Mesocephalon. The limits of the mesocephalon cannot readily 

 be defined, as it is continuous with the crura cerebri above, with the crura 

 cerebelli behind, with the medulla oblongata below, and with the cerebellum 

 itself above and behind. Many treat of it as one mass with the medulla 

 oblongata. We shall consider it as composed of the tubercula quadri- 

 gemina and the pons Varolii. 



The corpm-a^ or tuhercula quadrigemina^ are four eminences seen imme- 

 diately behind the third ventricle. A transverse furrow separates them 

 into an anterior, the nates ^ and a posterior pair, the testes; a longitudinal 

 furrow along the median line divides the right and left pairs from each 

 other. The pineal body rests in the anterior extremity of the longitudinal 

 depression. 



The posterior crura of the corpora quadrigemina are connected with the 

 cerebellum by two columns of white matter, one of which passes into the 

 central white substance of each cerebellar hemisphere ; these are the pi'o- 

 cessus cerebelli ad testes. They enter into the formation of the crura cerebelli. 

 The interval between the two processus cerebelli is occupied by a horizon- 

 tal stratum of nervous matter called the valve of Vieussens, or of the fourth 

 ventricle. 



The pons Varolii^ or great commissure of the cerebellum, is somewhat 

 square, and placed obliquely on the cuneiform process between the cere- 

 brum and the cerebellum. The fourth ventricle, the aqueduct of Sylvius, 

 and the corpora quadrigemina are on its superior and posterior surface. 

 Its superior extremity receives the crura cerebri, which it surrounds like a 

 ring, and is hence sometimes called the annular protuberance ; the crura 

 cerebelli are attached to its sides, and the medulla oblongata to its lower 

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