THE OPTIC NERVES. 517 



Real Origin of the Optic Nerves. The fibres of the optic nerves, in 

 man, as shown by the above description, cannot all be distinctly traced 

 in a direct manner to the tubercula quadrigemina ; but those of one 

 root are evidently connected with the corpus geniculatum externum, 

 while those of the other pass to the corpus geniculatum internum, and 

 through the intervention of that body alone reach the anterior quadri- 

 geminal tubercule. And yet the data derived from comparative anatomy, 

 as well as the results of experiments upon the tubercula quadrigemma, 

 show that in the lower animals these bodies are the real sources of the 

 optic nerves. In all the mammalian quadrupeds the optic nerves are 

 readily seen to have their direct origin in the tubercula quadrigemina. 

 In the birds, reptiles, and fish these bodies are divided only into two 

 symmetrical prominences by a shallow, longitudinal, median furrow ; 

 and in these classes, accordingly, they are called the " tubercula bigem- 

 ina." But they are of comparatively larger size than in the mammalians, 

 and give origin still more distinctly to the optic nerves. Furthermore, 

 their destruction, as shown by the united testimony of all observers, 

 produces at once a loss of sight, although the remaining parts of the 

 brain be left uninjured; while it is certain, on the other hand, that 

 both the hemispheres and the optic thalami may be removed without 

 destroying sensibility to light. Even in man, according to the obser- 

 vations of Yulpian, the optic thalami may be the seat of extensive 

 lesions, from hemorrhage or softening, without any sensible disturb- 

 ance of the power of vision. 



The apparent variation in man from the general type, in respect to 

 the origin of the optic nerves, is most readily explained by the variation 

 in the comparative size of the tubercula quadrigemina and the optic 

 thalami. In the inferior vertebrate animals, namely, fish and reptiles, 

 the tubercula quadrigemina or their representatives are very large, and 

 the optic thalami are either wanting or so slightly developed as to make 

 their significance uncertain. In birds the optic thalami are present, but 

 are still inferior in size to the tubercula bigemina. In mammalians 

 they increase in size in the ascending series, but the tubercula quadri- 

 gemina are still, in such animals as the dog and cat, comparatively con- 

 spicuous, and, throughout this class, consist of four tubercles instead 

 of two. In man the optic thalami are very much larger than the tuber- 

 cula quadrigemina, which are reduced altogether to a secondary grade. 

 The corpora geniculata probably represent in man portions of gray 

 substance included, in the lower animals, in the tubercula quadrigemina 

 or bigemina ; but which in the human brain are crowded outward and 

 backward by the increased development of the optic thalami, and there- 

 fore appear as appendages of these bodies. 



Physiological Properties of the Optic Nerves. The optic nerves, 

 like the olfactory, are nerves of special sense, and may be regarded as 

 tracts of fibres connecting the gray matter of the cerebrum with the 

 retinal expansion of the globe of the eye. They are destitute of sensi- 

 bility to tactile or painful impressions, and convey from without inward 



