354 INNERVATION. [CHAP. XI. 



There are many instances on record in which blindness was coin- 

 cident with pathological alteration of structure in one or both quadri- 

 geminal tubercles. In some of the cases where the lesion extended 

 to parts seated beneath the tubercles, disturbed movements were 

 observed, as in the experiments above related. 



We are ignorant of the object of the extensive connections of the 

 optic tracts with the tuber cinereum, the crura cerebri, and the 

 corpora geniculata; but these points are highly worthy of future 

 inquiry, especially with reference to the office of these last-named 

 bodies, which is at present involved in much obscurity. Many of 

 the fibres of the optic tracts are undoubtedly commissural between 

 the corresponding points of opposite sides, and exist when those 

 which form the optic nerves are deficient. 



We see, then, in the quadrigeminal tubercles, centres, which, 

 whatever other functions they may perform, have a sufficiently ob- 

 vious relation to the optic nerves, the eye, and the sense of vision. 

 This is clearly indicated by anatomical facts, by the results of 

 experiment, and by the phenomena of disease. These bodies may, 

 therefore, be justly reckoned as special ganglia of vision ; and we are 

 led to seek for similar centres in connexion with the other senses. 

 The olfactory processes seem very probably to perform a similar 

 office in reference to the sense of smell. Their structure, their 

 relation to the olfactory nerves, and their direct proportion of bulk 

 to that of these nerves, and to the development of the olfactory 

 apparatus, place this question beyond all doubt. It is not so easy 

 to determine the special ganglia of hearing; but the olivary bodies, 

 or the small lobules connected with the crura cerebelli called by 

 Reil the flocks, may be referred to as bearing a sufficiently close 

 anatomical relation to the auditory nerve to justify our regarding 

 either of them as well calculated to perform this function. And, 

 with respect to touch, the ganglia on the posterior roots of the 

 spinal and the fifth nerves may perhaps be considered in the same 

 light ; for this sense being diffused so universally, in various de- 

 grees, over the whole surface of the body, and being seated in a 

 great number of different nerves, would need ganglia in connexion 

 with all those nerves which are adapted to the reception of tactile 

 impressions. The analogous sense of taste has its ganglia in those 

 of the glosso-pharyngeal and the fifth.* 



* It may be urged against this conjecture respecting the functions of the 

 ganglia of the spinal nerves and the fifth, that the analogy between these 

 bodies and the quadrigeminal tubercles is incomplete, inasmuch as the optic 

 nerves are probably implanted in the latter, but the nerves of touch merely 



