722 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 tubercula 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 centres 

 for the movements of the iris, there is little to be said, except that their office is inti- 

 mately connected with 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 

 and 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-demonstra- 

 tions, of removing the optic lobe on one side from a pigeon, with the result just men- 

 tioned. 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 re- 

 moving 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 feu- 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 have long been regarded 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; but the experiments of Ferrier upon monkeys (1875) and of Dalton upon dogs 

 (1881) have demonstrated the remarkable fact that destruction of the angular convolution 

 of the cerebrum upon both sides produces total blindness. Destruction of this convolu- 

 tion upon one side was found to produce blindness of the eye upon the opposite side, while 

 the sight in the eye upon the same side was apparently unaffected. In all of the experi- 

 ments referred to, the crossed action of lesions of the angular convolution was very dis- 

 tinct; but in certain cases of affections of vision in the human subject, due to lesion of the 

 brain, which will be referred to more fully in connection with the question of decussa- 

 tion of the optic nerves, the injury produced loss of sight in one vertical half of the ret- 

 ina in either eye. 



Ganglion of the Tuber Annular e. 



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 



