14S 



ANATOMY OF VERTEBRATES. 



126 



a - 



Optic cliiasma ; Man. ecu. 



127 



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

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



fibres arc 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 cliiasma 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 cliiasma, 

 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 cliiasma. The hinder commissure is more common, and 

 appears as a little trenial border of the cliiasma, 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, 

 by 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 iieu- 

 rilemma, which sends processes 

 from its inner surface : 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 f arteria centralis retinas.' 



Atrophied right optic nerve and tract ; Human, ecu. 



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. 



