7 22 NERVOUS SYSTEM. 



Tubercula Quadrigemina. 



These little bodies, sometimes called the optic lobes, are rounded eminences, two 

 upon either side, situated just below the third ventricle. The anterior, called the nates, 

 are the larger. These are oblong and of a grayish color externally. The posterior, 

 called the testes, are situated just behind the anterior. They are rounded and are rather 

 lighter in color than the anterior. Both contain gray nervous matter in their interior. 

 They are the main points of origin of the optic nerves and are connected by commissu- 

 ral fibres with the optic thalami. In birds, the tubercles are two in number, instead of 

 four, and are called tubercular bigemina. 



It is probable that the tubercula quadrigemina are inexcitable and insensible. When 

 pain and convulsive movements have apparently followed their mechanical irritation in 

 living animals, these phenomena have probably been due to excitation or stimulation of 

 the motory or sensory commissural fibres which pass beneath them. 



As regards the function of the optic lobes, aside from their action as reflex nervous 

 centres for the movements of the iris, there is little to be said, except that they preside 

 over the sense of sight. They are easily reached and operated upon in birds, where they 

 are very large, and their extirpation is followed by total loss of sight, as well as abolition 

 of the reflex movements of the iris. In birds and in those mammals in which they have 

 been operated upon, their action in vision is crossed ; i. e., when the lobe is removed upon 

 one side, the sight is lost in the opposite eye, vision in the eye upon the same side being 

 unimpaired. We have long been in the habit, in class-demonstrations, of removing the 

 optic lobe on one side from a pigeon, with the result just mentioned. The operation is 

 quite simple : A part of the skull is removed by the side of one hemisphere, and the 

 optic lobe is seen, in the form of a large, white tubercle, between the posterior portion 

 of the cerebrum and the cerebellum. A little slit is then made in its capsule, and the 

 interior is broken up carefully with a delicate forceps. The animal generally recovers 

 from the operation, blinded in the eye upon the opposite side. In removing the portion 

 of the skull, it is well not to go too far back, as there is then danger of wounding the 

 great venous sinus and complicating the operation by haemorrhage. 



In treating of the special sense of sight, we shall see that the decussation of the 

 optic nerves is more complex in man than in birds, in which the nerve from one optic 

 lobe passes totally and exclusively to the eye upon the opposite side. In man, most of 

 the fibres of the optic nerve from one side pass to the eye upon the opposite side ; but a 

 few fibres pass to the eye upon the same side, a few connect the tubercles upon the two 

 sides, and a few connect the two. eyes. It is not known whether or not, in man, the 

 action of the tubercles in vision is exclusively crossed, as it appears to be in most of the 

 inferior animals. 



The optic lobes undoubtedly serve as the sole centres presiding over the sense of 

 sight, and not merely as avenues of communication of this sense to the cerebral hemi- 

 spheres. A positive proof of this proposition lies in the fact that the sense of sight is 

 preserved after complete removal of the cerebrum, provided that injury of the tuber- 

 cles have been carefully avoided. 



We shall say nothing, in this connection, with regard to t>he movements of the iris, 

 except that the reflex action by which the size of the pupil is modified is effected through 

 the optic lobes as nerve-centres. The mechanism of the movements of the iris and their 

 regulation through nervous action are questions of great interest and are somewhat com- 

 plex. We have already treated of them to some extent, in connection with the physi- 

 ology of the third pair of nerves, and they will be considered still more fully in the sec- 

 tion upon the special sense of sight. 



Ganglion of the Tuber Annulare. 



The tuber annulare, called the pons Varolii or the mesocephalon, is situated at the 

 base of the brain, just above the medulla oblongata. It is white externally and contains 



