THE OPTIC CHIASM A 319 



and there in space? The traditional explanation of them is based upon 

 a certain peculiarity of the mammalian optic nerves : 



The Optic Chiasma in Man and Other Vertebrates — In the verte- 

 brates the optic nerves from the two eyes never enter directly the respec- 

 tive sides of the brain. Instead, they come together beneath the brain 

 and cross over or through each other in an x-shaped structure called the 

 optic chiasma (Figs. 21, 70; pp. 47, 172). From this, they continue to the 

 brain as the 'optic tracts'. In all vertebrates from the lampreys to the birds 

 inclusive, all of the optic nerve fibers from one eye cross over in the chias- 

 ma to form the optic tract of the other side, so that each eye is connected 

 only with the opposite half of the brain. This is called 'total decussation'. 

 Similar decussations are very numerous among the fiber tracts of the 

 spinal cord and brain stem, and there is no discernible reason for any of 

 them — they apparently just happened in embryos during the early evolu- 

 tion of the vertebrates, and became genetically fixed in the group. A 

 vague sort of case might be made out for having the primitive verte- 

 brate's left eye connected with the part of the central nervous system 

 which controls the muscles of the right side, for these would be most 

 important in turning the animal to face a light coming from the left. 

 But, the left eye is connected with the right brain which owing to decus- 

 sations in the motor tracts, controls the muscles of the left side. If we 

 stick to our teleological guns, we are then forced to believe that the first 

 vertebrates were negatively phototropic, which is most improbable. 



In the optic chiasma of the mammals, and only in the mammals, an 

 important modification occurs. In these animals the decussation of the 

 optic nerves is partial: some of the afferent fibers from each retina fail 

 to cross over, and hence enter the optic tract on that same side. In man, 

 the proportion is just about fifty per cent, with half of the macular fibers 

 as well as half of the extra-macular ones remaining uncrossed (Fig. 21a). 

 The fibers from the nasal half of each retina are the only ones which 

 decussate; and although there is no visible evidence of it, there is a 

 vertical line neatly bisecting each human retina, which is the boundary 

 between the retinal area connected with the same side of the brain and 

 that connected with the other side. 



In all mammals, the relative number of uncrossed fibers is closely pro- 

 portional to the degree of frontality. It is about one-eighth to one-sixth 

 of the whole in the horse, one-fifth in the rat and in the common 

 opossum, one-fourth in the dog and the Australian bushy-tail opossum 

 (Trichosurus vulpecula), one-third in the cat, and reaches a maximum of 



