540 A MANUAL OF VETERINARY PHYSIOLOGY 



and the anterior corpus quadrigeminum. The most characteristic 

 naked-eye feature in connection with the nerves is the fusion 

 which occurs. The nerves from the two eyes meet on the base 

 of the brain, and form. what is known as the optic chiasma. They 

 again separate and pass to their respective sides of the brain. 

 This crossing of the optic nerves is a question of great importance, 

 and whether the crossing is complete or incomplete depends 

 upon the animal and the type of eye. In fishes the nerves cross 

 and remain quite distinct : one may even pass through a slit 

 in the other, but neither gives fibres to the other. In birds the 

 fibres meet, interlace, alternate, but the decussation is complete 

 — viz., the fibres of the right eye all pass to the left brain, and 

 vice versa. The law, in fact, regarding the decussation of the 

 fibres of the optic nerve is that in all vertebrates below 

 mammals — i.e., in fishes, amphibia, reptiles, and birds — there 

 is total decussation. In all animals higher than these decussa- 

 tion is incomplete ; at first only a small number, and gradually, 

 as monkeys and man are approached, a considerable number of 

 fibres pass from the retina to the same side of the brain. In 

 the horse one-sixth of the fibres are said to be direct, in man 

 three-fifths. 



The question of decussating and direct fibres hinges largely, 

 if not entirely, on the position of the eyes in the animal's head. 

 When, for instance, the eyes are laterally placed, and any attempt 

 to look forward is out of the question, decussation is complete ; 

 but as a certain amount of forward as well as lateral vision 

 becomes established, a proportion of direct fibres in the nerve 

 occurs, until at last the animal with perfect forward and imperfect 

 lateral vision is reached, when more than half of the fibres are 

 direct. Nor is this a coincidence ; on the other hand, it is 

 intimately connected with the question of the visual path, which 

 has been traced into the higher regions of the brain. 



In fishes the full extent of the visual path is reached at the mid- 

 brain, as these animals possess no cerebrum ; but as the scale is 

 ascended, and the cortical centres develop, the visual path extends 

 upward from the mid-brain to the cerebrum, where, in its occipital 

 portion, a visual region arises, fibres from the external geniculate 

 body being projected on to the occipital lobe. Here we meet with 

 another example of the unity of type of the nervous system. The 

 first neurone exists between the retina and the mid-brain, the 

 optic fibres being axons of the nerve cells of the retina. By 

 synapses these are connected with a second neurone running from 

 mid-brain to cerebral cortex. The area connected with vision in 

 the higher mammals can be accurately mapped in the calcarine 

 fissure of the occipital lobe by following a white line, which can be 

 readily traced, known as the line of Gennari. This line is a layer 

 of nerve fibres of a special kind, lying midway between the surface 

 and the white matter of the brain. The area thus mapped out is 



