Comparative Anatomy of the Brain. 603 



it is in the embryos of birds, reptiles and fishes. But while in these an- 

 imals it becomes folded, or laminated, into a pile of plates, as in fig. 

 291, in man it is doubled over once, and its edges joined together to 

 form a tube. There appear to be intermediate stages between these two 

 forms. In cetacea the nerve is a cylinder in which longitudinal parti- 

 tions are arranged converging toward the center, one edge being fas- 

 tened to the external sheath, tlfe whole thus forming a bunch of longi- 

 tudinal triangular chambers. In others, the nerve is composed of lon- 

 gitudinal canals enclosed within the external sheath of dura* mater. 

 Either of these forms might result from the folding of the optic nerve 

 of the fish into a cylinder, the circumference of which would retain lon- 

 gitudinal corrugations. Further modifications of these might convert 

 them into tubules, or abolish them altogether, leaving the nerve a plain 

 cylinder, as in man. 



There is no decussation of the optic nerves in the case of the hags 

 and lampreys, but each nerve connects only with the eye on its own side. 

 In the foetal development of the vertebrates it is the same ; the nerve at 

 first connecting the eye with the vesicles of the third ventricle on its 

 own side ; later, the nerve fibres gradually extend over to the opposite 

 side of the brain and form the fusion with the opposite nerve. These 

 facts are sufficient evidence that originally the nerve fibres did not cross, 

 and that the habit was taken up, from some cause, by the ancestor, or 

 ancestors, of the present races. That the process was gradual is also 

 proved by comparative anatomy, because, in different animals there are 

 different degrees of perfection in the chiasm. Thus it appears that in 

 the Skate (fig. 285 ) the nerves are connected together by a commissure. 

 In the teleostean fishes the nerves cross, but there is no commissure or 

 mingling of the fibres. (See figs. 81 and 289.) In the mammals, how- 

 ever, there is both a crossing and a fusion. (Figs. 270, 317, 336, 337, 

 &c.) If this decussation had not taken place the right eye would have 

 been in connection with the left side of the body exclusive^, because, 

 on account of the decussation of the fibres of the spinal axis in the 

 medulla oblongata, the right side of the brain, with which the right eye 

 was first connected, is the organ of the left side of the body. It is 

 obvious that while the right eye can help the left side, it is more effec- 

 tual as a guide to the movements of the right side than of the left ; 

 and the left eye can better serve the left side than the right. Hence the 

 economy of the crossing of optic nerves, by which each e} T e is put into 

 connection with its own side of the body. That which is a real econ- 

 omy in the operation of natural energies, is in constant process of adop- 

 tion in the organic world. The history of differentiation is a history of 

 short cuts, and the elimination of indirect methods. The necessary 

 tendency would be, in the case of the eyes, to rectify by a new decussa- 



