OPTIC NERVES. 



769 



nerves thus united ; and for these reasons the 

 physiology of the chiasma is invested with 

 uncommon interest. 



The existence of a chiasma is not general 

 throughout the animal series, and even when 

 present it exhibits much diversity of appearance 

 and structure in different classes. A brief ex- 

 position of some of its more striking varieties, 

 in animals, will probably constitute the best 

 introduction to the study of the chiasma in man. 



Invertebrata. In the invertebrate classes 

 nothing like a chiasma has been demonstrated, 

 nor has any mutual crossing of the special optic 

 nerves been proved to exist. The nerves winch 

 are furnished to the compound eyes of insects 

 and crustaceans pass in a direct course to their 

 destination ; the same remark applies to the 

 nerves from which the lens-eyes of insecta, 

 arachnida, Crustacea, and mollusca, derive their 

 sensibility ; and it may be presumed that the 

 nerves which supply the simple eye-dots of 

 annelida and other inferior animals are similarly 

 circumstanced. 



Osseous fish. In osseous fish the optic 

 nerves generally cross each other at an acute 

 angle, in such manner that the nerve which 

 comes from the right side of the brain goes 

 distinctly to the left eye, and vice versa : at the 

 point of decussation the nerves lie one over the 

 other; they are usually flattened at this spot, and 

 closely joined together, but this junction is 

 effected by means of cellular or fibrous ad- 

 hesions only, as no intermingling of the nervous 

 filaments takes place, and the nerves themselves 

 can be isolated without injury to their proper 

 structure (Jig. 407). 



Cartilaginous fish. In cartilaginous fish K 

 well-marked chiasma occurs. In this class the 

 junction of die two optic nerves is no longer 

 effected by means of mere cellular adhesions, 

 as in the osseous fish, but a perfect union of the 

 proper substance of the nerves constitutes a 

 true chiasma. The optic nerves arise each 

 from the corresponding optic lobe chiefly ; they 

 quickly converge, and soon become confounded 

 with each other in the chiasma; and so inti- 

 mate is their connection, that anatomists possess 

 little information as to the exact arrangement 

 of the nervous filaments in this structure 



(fig- 410). 



Birds. In birds the chiasma is large, being 

 proportional to the size of the optic nerves; by 

 a little management its organization can be 

 accurately demonstrated. Maceration for a 

 few days in spirits hardens this structure suffici- 

 ently to enable the operator to strip off the 

 neurilemma, and then, even without the aid ofa 

 lens, the chiasma may be seen to consist chiefly 

 of laminae. Forcible extension of the optic 

 nerves, in such a manner as to tear through the 

 superficial stratum of the chiasma on its lateral 

 aspect, greatly facilitates the examination. 



The lamina; originate in the tractus opticus, 

 and appear to spring from the inner part alone 

 of that mass ; they gain the chiasma, and here 

 those derived from opposite sides of the brain 

 form a reciprocal interlacement. A perfect and 

 regular decussation of the inner filaments of the 

 two tractus optici thus takes place in the 



VOL. III. 



chiasma, in such manner that a large proportion 

 of the tractus of one side is evidently traceable 

 to the opposite optic nerve, and vice versa: 

 but the outer part of each tractus opticus con- 

 tinues on to form the outer part of the optic 

 nerve of its own side, and has no concern in the 

 formation of the decussating laminae. 



The number of laminae in the chiasma of 

 different birds is subject to some variety, but 

 in the entire class, without exception, the lami- 

 nated structure prevails (fig. 417). 



Fig. 417. 



Chiasma of the common fowl. (After Midler.) 



a, a, optic nerves ; b, chiasma, dissected so as 

 to shew its decussating lamina ; c, c, tractus 

 optici. 



Amphibia and reptiles. In amphibia and 

 reptiles a laminated chiasma, somewhat similar 

 to that just described in birds, occurs; but 

 the decussating laminae are very variable in 

 number, and in general much fewer than in 

 birds. 



Thus, in Amphisboena, according to Miiller, 

 there are only five laminae in all, two trom one 

 side, and three from the other ; and in lacerta 

 ocellata, according to the same authority, as 

 many as eight have been counted, four on either 

 side (fig. 418). In some reptiles the posterior 

 part of the chiasma is strictly commissural, the 

 inner part of each tractus opticus being appro- 

 priated to the formation of a band-like com- 

 missure: in Amphisboena a triangular space 

 separates this band from the remainder of the 

 chiasma (fig- 418). 



A. 



Fig. 418. 



Chiasma in Amphisbama. ( After Miiller.) 



A, section of chiasma to exhibit the decussating; 

 laminae, of which there are three from one side and 

 two from the other. 



B, chiasma seen from below. 



a, a, optic nerves ; b, b, tractus oplici ; c, com- 

 missural band ; d, triangular space ; f, true chi- 

 asma. 



Mammalia and man. In mammalia and man 

 the chiasma is no longer laminated, and great 

 difficulty occurs in attempts to display its real 

 structure. 



The older anatomists were evidently unable 

 to trace the filaments of the human optic nerves 

 satisfactorily through the chiasma, and in con- 

 sequence they relied either on pathological 

 facts, or the results of experiments, or the data 

 furnished by comparative anatomy, to determine 

 the mutual relations of the second pair in this 



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