STRUCTURE OF THE CEREBRUM. 599 



anteriora) 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 pro- 

 portion of the cerebral mass; at first, they are only two in number, as in the 

 lower mammalia, and hollow in their interior. 



These bodies receive, from below, 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 cerebel- 

 lum, 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 trans- 

 versely grooved lobule of gray matter prolonged from the anterior border of 

 the cerebellum; this is called by the Italian anatomists 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 exter- 

 num and internum. They are placed one on the outer and one on the inner 

 side of each opic tract. In this situation, the opic 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 commissural 

 fibres, which connect together the two hemispheres. 3. Longitudinal commis- 

 sural 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 originate in the columns of the cord and 

 medulla oblongata, and enter 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 pyramid, and, ascending, pass mainly through the centre of the 

 striated body; those on the opposite surface of the crus, which form the 

 tegmentum, are derived from the posterior pyramid and fasciculi teretes; as 

 they ascend, 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. Some of the diverging 

 fibres end in the cerebral ganglia, whilst others pass through and receive addi- 

 tional fibres from them, and as they emerge, radiate into the anterior, middle, 

 and posterior lobes of the hemisphere, decussating again with the fibres of the 

 corpus callosum, before passing to the convolutions. These fibres have received 

 the name of corona radiata. 



The transverse commissural fibres connect together the two hemispheres across 

 the middle line. They are formed by the corpus callosum, and the anterior and 

 posterior commissures. 



