TUBERCULA QUADRIGEMIXA. 435 



the sense of sight ; on which account they are also known by the 

 name of the " optic ganglia." Their development corresponds very 

 closely with that of the external organs of vision. Thus, they are 

 large in fish, reptiles, and birds, in which the eyeball is for the 

 most part very large in proportion to the entire head; and are small 

 in quadrupeds and in man, where the eyeball is, comparatively 

 speaking, of insignificant size. Direct experiment also shows the 

 close connection between the tubercula quadrigemina and the sense 

 of sight. Section of the optic nerve at any point between the 

 retina and the tubercles, produces complete blindness ; and destruc- 

 tion of the tubercles themselves has the same effect. But if the 

 division be made between the tubercles and the cerebrum, or if the 

 cerebrum itself be taken away while the tubercles are left un- 

 touched, vision, as we have already seen, still remains. It is the 

 tubercles, therefore, in which the impression of light is perceived. 

 So long as these ganglia are uninjured and retain their connection 

 with the eye, vision remains. As soon as this connection is cut 

 off) or the ganglia themselves are injured, the power of vision is 

 destroyed. 



The tubercula quadrigemina not only serve as nervous centres 

 for the perception of light, but a reflex action also takes place 

 through them, by which the quantity of light admitted to the eye 

 is regulated to suit the sensibility of the pupil. In darkness and 

 in twilight, or wherever the light is obscure and feeble, the pupil 

 is enlarged by relaxation of its circular fibres, so as to admit as 

 large a quantity of light as possible. On first coming into a dark 

 room, accordingly, everything is nearly invisible ; but gradually, 

 as the pupil dilates and as more light is admitted, objects begin to 

 show themselves with greater distinctness, and at last we can see 

 tolerably well in a place where we were at first unable to perceive 

 a single object. On the other hand, when the eye is exposed to an 

 unusually brilliant light, the pupil contracts and shuts out so much 

 of it as would be injurious to the retina. 



The above is a reflex action, in which the impression received by 

 the retina is transmitted along the optic nerve to the tubercula 

 quadrigemina. From the tubercles, a motor impulse is then sent 

 out through the motor nerves of the eye and the filaments dis- 

 tributed to the iris, and a contraction of the pupil takes place in 

 consequence. The optic nerves act here as sensitive fibres, which 

 convey the impression from the retina to the ganglion ; and if 

 they be irritated in any part of their course with the point of a 



