143 



ANATOMY OF VERTEBRATES. 



126 



Optic cliiasma ; Man. ecu. 



forward, converging and meeting beneath the brain at their con- 

 fluence, called ' chiasma opticum,' a, b. The fasciculi of primitive 

 fibres are here arranged as shown in fig. 

 126. The outer ones, b, pass onward to 

 form the outer side of the nerve a, the middle 

 fasciculi cross the chiasma obliquely, and, 

 after decussating the corresponding fasciculi 

 of the other tract, contribute to the formation 

 of the opposite nerve : the inner fasciculi 

 curve across the back part of the chiasma, 

 and are continuous with the corresponding 

 fasciculi of the opposite tract, being strictly ' commissural : ' a 

 similar arrangement prevails with a few fasciculi at the fore part 

 of the chiasma. The hinder commissure is more common, and 

 appears as a little tamial border of the chiasma, in some Mammals, 

 down to the rodents. Pathology gives evidence of a partial 



decussation, in some instances, 

 as in the preparation, fig. 127 ; 

 in which the right optic nerve, 



a, was atrophied ; the left one, 



b, healthy; with a partially 

 wasted left optic tract, c, while 

 the right, d, retained more of 

 its normal size. 1 



The Mammalian chiasma 

 ceases to show the laminated 

 arrangement (vol. ii. p. 122, 

 fig. 47) common in Birds and 

 Reptiles. The nerve, beyond 

 the chiasma, has a strong neu- 

 rilemma, which sends processes 

 in some, e. g. Cetacea, converging as lon- 

 gitudinal septa from the circumference to the centre of the nerve ; 

 in most forming longitudinal canals for the neurine, and giving it 

 the character of a cylindrical aggregate of tubes. This is enclosed 

 in a sheath of dura mater, extending to the sclerotic, into which 

 it is partly-continued, where the nerve pierces that coat of the 

 eye-ball. Another peculiarity is seen in the small artery running 

 along the centre of the nerve, and ramifying upon its terminal 

 expansion as the • arteria centralis retinas.' 



Atrophied right optic nerve and tract ; Human, cci 



from its inner surface 



1 There have been cases, however, where the tract of the same side as the atrophied 

 nerve showed more wasting than that of the opposite side. 



