OPTIC NERVES. 39 



from the posterior, and the external white root, which is the 

 larger, from the anterior tuberculum. The gray root is situ- 

 ated in front of and above the optic commissure, and is a de- 

 pendence of the gray matter which covers the internal surface 

 of the optic thalamus. It arises from the gray matter consti- 

 tuting the anterior floor of the third ventricle, in the form 

 of delicate filaments which join the optic nerves at this 

 point. 1 



The apparent origin of these nerves is from the tubercula 

 quadrigemina, receiving filaments from the corpora genicu- 

 lata, the optic thalami, the peduncles of the cerebrum, the 

 anterior substantia perforata, the tuber cinereum, and the 

 lamina terminalis. It has thus far been found impossible to 

 trace all these roots to their true origin in the cerebral sub- 

 stance ; but experiments upon the lower animals, in which it 

 has been shown that the sense of sight is completely abol- 

 ished by destruction of the tubercula quadrigemina (bigemi- 

 na, in birds), show that the origin of the filaments that pre- 

 side over vision is, in all probability, from these bodies. 



The two principal roots unite above the external corpus 

 geniculatum, forming a flattened band, which takes an oblique 

 course around the under surface of the crus cerebri to the 

 optic commissure. This is usually called the optic tract, in 

 contradistinction to the optic nerve, which is described as 

 arising from the optic commissure. 



The optic commissure, or chiasm, is situated just in front 

 of the corpus cinereum, resting on the olivary process of the 

 sphenoid bone. As its name implies, this is the point of union 

 between the nerves of the two sides. At the commissure, the 

 fibres from the optic tracts take three directions ; and, in addi- 

 tion, the commissure contains filaments passing from one eye 

 to the other, which have no connection with the optic tracts. 

 The four sets of fibres in the optic commissure are as fol- 

 lows : 



1. Decussating fibres, passing from the optic tract upon 



* SAPPET, Traite cTanatomie, Paris, 1871, tome iii., p. 251. 



