CORPORA QUADRIGEMINA VALVE OF VIEUSSENS. 529 



The corpora or tubercula quadrigemina the optic lobes are four rounded emi- 

 nences placed in pairs, two in front and two behind, separated from another by a 

 crucial depression. They are situated immediately behind the third ventricle 

 and posterior commissure, beneath the posterior border of the corpus callosum, 

 and above the iter e tertio ad quartum ventriculum. The anterior pair, the nates, 

 are the larger, oblong from before backwards, and of a gray color. The posterior 

 pair, the testes, are hemispherical in form, and lighter in color than the preceding. 

 They are connected on each side with the thalarnus opticus, and commencement 

 of the optic tracts, by means of two white prominent bands, termed brackia. Those 

 connecting the nates with the thalamus (brachia anteriord) are the larger, and pass 

 obliquely outwards. Those connecting the testes with the thalamus are called the 

 brachia posteriora. Both pairs, in the adult, are quite solid, being composed of white 

 matter externally, and gray matter within. These bodies are larger in the lower, 

 animals than in man. In fishes, reptiles, and birds, they are only two in number, 

 are called the optic lobes, from their connection with the optic nerves, and are 

 hollow in their interior ; but in mammalia, they are four in number, as in man, and 

 quite solid. In the human foetus, they are developed at a very early period, and 

 form a large proportion of the cerebral mass; at first, they are only two in number, 

 as in the lower mammalia, and hollow in their interior. 



These bodies, from below, receive white fibres from the olivary fasciculus or fillet ; 

 they are also connected with the cerebellum, by means of a large white cord on 

 each side, the processus ad testes or superior peduncles of the cerebellum, which 

 pass up to the thalami from the tubercula quadrigemina. 



The valve of Vieussens is a thin translucent lamina of medullary substance, 

 stretched between the two processus e cerebello ad testes ; it covers in the canal 

 leading from the third to the fourth ventricle, forming part of the roof of the latter 

 cavity. It is narrow in front, where it is connected with the testes ; and broader 

 behind, at its connection with the vermiform process of the cerebellum. A slight 

 elevated ridge, the frenulum, descends upon the upper part of the valve from the 

 corpora quadrigemina, and on either side of it may be seen the fibres of origin of 

 the fourth nerve. Its lower half is covered by a thin transversely grooved lobule 

 of gray matter prolonged from the anterior border of the cerebellum ; this is called 

 the linguetta laminosa. 



The corpora geniculata are two small, flattened, oblong masses, placed on the 

 outer side of the corpora quadrigemina, and on the under and back part of each 

 optic thalamus, and named, from their position, corpus geniculatum externum and 

 corpus geniculatum mternum. They are placed one on the outer and one on the 

 inner side of each optic tract. In this situation, the optic tract may be seen 

 dividing into two bands, one of which is connected with the external geniculate body 

 and nates, the other being connected with the internal geniculate body and testis. 



/Structure of the cerebrum. The white matter of each hemisphere consists of 

 three kinds of fibres. 1. Diverging or peduncular fibres, which connect the 

 hemisphere with the cord and medulla oblongata. 2. Transverse comrnissural 

 fibres which connect together the two hemispheres. 3. Longitudinal commissural 

 fibres, which connect distant parts of the same hemisphere. 



The diverging or peduncular fibres consist of a main body, and of certain accessory 

 fibres. The main body of fibres originates in the columns of the cord and medulla 

 oblongata, and enters the cerebrum through the crus cerebri, where they are 

 arranged in two bundles, separated by the locus niger. Those fibres which form 

 the inferior or fasciculated portion of the crus are derived from the anterior 

 P3 r ramid, and, ascending, pass mainly through the centre of the striated body ; those 

 on the opposite surface of the crus, which form the tegrnentum, are derived from 

 the posterior pyramid and fasciculi teretes ; ascending, they pass, some through the 

 under part of the thalamus, and others through both thalamus and corpus striatum, 

 decussating in these bodies with each other, and with the fibres of the corpus 

 callosum. The optic thalami also receive accessory fibres from the processus ad 

 testes, the olivary fasciculus, the corpora quadrigemina, and corpora geniculata 

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