438 THE BRAIN. 



the same object, the optic nerves decussate with each other in such a 

 manner as to form a connection between the two opposite sides, as 

 well as between each tubercle and retina of the same side. (Fig. 

 149.) This decussation, which is somewhat complicated, takes place 

 in the following manner. From each optic tubercle three different 

 bundles or " tracts" of nervous fibres are given off. One set passes 

 across transversely at the point of decussation, and, turning back- 

 ward, terminates in the tubercle of the opposite side ; another, cross- 

 ing diagonally, continues onward to the opposite eyeball ; while a 

 third passes directly forward to the eyeball of the same side. A 

 fourth set of fibres, still, passes across in front of the decussation, 

 from the retina of one eye to that of the opposite side. We have, 

 therefore, by this arrangement, the two retina^ as well as the two 

 optic tubercles, connected with each other by commissural fibres ; 

 while each tubercle is, at the same time, connected both with its 

 own retina, and with that of the opposite side. It is undoubtedly 

 owing to these connections that when, in the human subject, the 

 eyes are directed in their proper axes, the two retinae, as well as 

 the two optic tubercles, act as a single organ. Yision is single, 

 therefore, though there are two images upon the retinas. Double, 

 vision occurs only when the eyeballs are turned out of their proper 

 direction, so that the parallelism of their axes is lost, and the image 

 no longer falls upon corresponding parts of the two retinae. 



TUBER ANNULARE. The collection of gray matter imbedded in 

 the deeper portions of the tuber annulare occupies a situation near 

 the central part of the brain, and lies directly in the course of the 

 ascending fibres of the anterior and posterior columns of the cord. 

 This ganglion is immediately connected with the functions of sensa- 

 tion and voluntary motion. We have already seen that these func- 

 tions are not destroyed by taking away the cerebrum, and that they 

 also remain after removal of the cerebellum. According to the ex- 

 periments of Longet, even after complete removal of the olfactory 

 ganglia, the cerebrum, cerebellum, optic tubercles, corpora striata 

 and optic thalami, and when nothing remains in the cavity of the 

 cranium but the tuber annulare and the medulla oblongata, the 

 animal is still sensitive to external impressions, and will still en- 

 deavor by voluntary movements to escape from a painful irritation. 

 The same observer has found, however, that as soon as the ganglion 

 of the tuber annulare is broken up, all manifestations of sensation 

 and volition cease, and even consciousness no longer appears to 



