554 THE CEREBRUM. 



anterior tubercles are much larger than the posterior, as may be seen in the 

 sheep, horse, and rabbit ; and hence the name nates, formerly applied to 

 the anterior, and tebtes to the posterior tubercles. In the brains of carnivora, 

 the posterior tubercles are rather the larger. In the foetus of man and 

 mammals these eminences are at first single on each side, and have 

 an internal cavity communicating with the ventricles. They are constant 

 in the brains of all vertebrate animals ; but in fishes, reptiles, and birds, 

 in which animals they receive the name of optic lobes, they are only two in 

 number, and hollow : in marsupialia and monotremata, they are also two in 

 number, but are solid. 



Optic tracts and corpora, geidculata. The optic tracts, which have already 

 been referred to in connection with the base of the cerebrum, are attached 

 to and embrace the under side of the corresponding peduncles, and 

 may be traced back to the thalami. Ench tract, somewhat cylindrical 

 towards the optic commissure, becomes flattened and broader as it approaches 

 the thalamus, and makes a bend as it turns round the peduncle to reach 

 the back part of that body. Near this bend, which is named the knee 

 (genu), and to the outer side of the corpora quadrigemina, are placed two 

 small oblong and flattened eminences connected with the posterior extremity 

 of the optic tract. They are two little masses of grey matter about the size 

 and shape of coffee-beans, placed one on the outer and one on the inner side 

 of the genu of the optic tract, and hence are named respectively corpus 

 geniculatum externum and internum. They send fibres into the optic tract 

 and also into the thalamns of the same side. 



The fibres of the optic tracts are therefore derived from three sources, 

 viz. , the thalamus, the tubercula quadrigemina, and the corpora geniculata. 



The processus a cerebello ad cerebrum are two large white cords extending 

 downwards and somewhat outwards from the corpora qnadrigemina to the 

 fore part of the cerebellum, and connecting the latter with the cerebrum. 

 They rest upon the crura cerebri, to which they are united, and between 

 them is the valve of Vieussens. 



The valve of Vieussens (velum medullare anterius), stretched between the 

 processus a cerebello ad cerebrum, is a thin layer of nervous matter, which 

 lies over the passage from the third to the fourth ventricle, and, lower down, 

 covers in a part of the fourth ventricle itself. It is narrow above, where it 

 is connected with the quadrigeminal bodies, and broader below, where it 

 is continuous with the median portion of the cerebellum. 



The upper portion of the valve is composed of white substance, but a few 

 transverse ridges of grey matter extend upon its lower half, as if they were 

 prolonged from the grey lamellae of the cerebellum with which the valve is there 

 continuous. From between the posterior quadrigeminal tubercles a slight 

 median ridge, named frmnulum, descends a little way upon the valve ; 

 and on the sides of this the commencing fibres of the fourth pair of 

 nerves pass transversely outwards. The back part of the valve is over- 

 lapped and concealed by the superior vermiform process of the cerebellum. 



INTERNAL STRUCTURE OF THE CEREBRUM. 



The cerebrum, like the rest of the encephalon, is composed of white and grey 

 substance, the white pervading nearly the whole of its extent, though more exclu- 

 sively composing its deeper parts ; the grey forming a covering of some thickness over 



