THE CEREBRUM. 581 



commissure of the optic nerves bounds them in front, and the 

 triangular cavity left by their divergence is closed up below by 

 the tuber cinereum, (pons Tarini) which surrounds the base of 

 the eminentiae mammillares h, fig. 224. Into these latter bodies 

 passes a band of white matter, which comes from the corpus 

 innominatum or lateral portion of the medulla oblongata. 



The Tubercula Quadrigemina, 



— (Lobi optici of animals, see fig. 225,) are four tubercles 

 placed upon the top of the pons, upon which they rest by their 

 lateral borders, and form an arch over the aqueduct of Sylvius. 

 — They form two pairs ; the superior called the nates, the infe- 

 rior the testes. These bodies are rather small and rudimental 

 in man, but large in the inferior animals. Tliey contain in their 

 interior some cineritious matter, and are covered on their outer 

 surface, by medullary fibres. Their direction is obliquely for- 

 wards and upwards. The posterior tubercles or testes are the 

 smaller, nearly hemispherical in shape, and are separated from 

 the anterior by a transverse groove. This groove is crossed by 

 another in the antero-posterior direction, which separates the two 

 bodies of the right, from those of the left side. 

 — Upon the testes terminates the valve of Vieussens, the rounded 

 column of each side called processus e cerebella ad testes, which 

 form the lateral boundaries of the fourth ventricle, and a trian- 

 gular medullary fasciculus from the side of the pons, which may 

 be traced upwards from the fascia innominata of the antero-lateral 

 column. The fibres from these sources pass up through the 

 tubercles, appear to be reinforced in the gray matter there, and 

 expand into the optic thalamus of each side, with which the 

 nates is continuous in an oblique direction, though partially sep- 

 arated from it by a slight groove. From the anterior extremity 

 of the nates, part some medullary fibres, which form a thin layer 

 over the corpus geniculatum externum of the corresponding thala- 

 mus, to assist in the formation of the optic nerve. A similar arm 

 of medullary matter passes, into the thalami, from either testis. 



Cerehntm. 

 — The cerebrum is that portion of the encephalic mass, which 

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