736 FUNCTIONS OF THE CEREBRUM [CH. L. 



which acts as the centre for the reflex; the higher we ascend the 

 animal scale, the path via the cortex becomes more permeable, of 

 greater value or even indispensable, and the reflexes through the 

 lower centres of less importance; not only so, but there are sub- 

 divisions of the visual cortical area, which correspond to different 

 regions of the retinae. 



We may in fact speak of the visuo-sensory field in the cortex as 

 the cortical retina upon which the impulses from the actual retina 

 in the eye are projected, in a manner analogous to the way in which 

 the field of vision is projected upon the actual retina. 



In the fishes which have no cortex cerebri, the optic lobes, 

 analogous to the C. quadrigemina, are the centres for vision. In 

 some fishes, a small number of the fibres of the optic nerve pass into 

 the geniculate body, which forms a cell-station on the road to the 

 posterior region of the cerebrum, where a primitive cortex begins to 

 appear. On ascending the animal scale, this group of fibres becomes 

 more and more abundant, and this part of the cortex becomes more 

 elaborate in structure. When we reach the monkeys, this part of 

 the brain is cut off from the rest to form a distinct occipital lobe by 

 the parieto-occipital fissure, which is frequently called the Affenspalte 

 (ape's split). In the lower monkeys this lobe is smooth (fig. 427, A, 

 p. 691), but as the great parietal association centres get larger with 

 increase of intelligence, the visuo-sensory area is pushed back, and 

 the lobe thrown into folds. In the highest apes, and in the lower 

 races of mankind, a good deal of the visuo-sensory sphere is still 

 seen on the external cerebral surface; but in the higher races, 

 most is pushed round on to the mesial surface. This calcarine area 

 is also named the striate area, because it is characterised by the 

 white stripe called the line of Gennari. 



Some animals have panoramic and others stereoscopic vision. 

 The former (mainly vegetable feeders) have eyes set laterally ; each 

 eye receives a different picture, and the decussation of the optic 

 nerves is complete; each eye sends impulses to the opposite hemi- 

 sphere. Animals with stereoscopic vision have the eyes, as in man, 

 in front, and the optic axes can be converged so that an object is 

 focussed with both eyes. This becomes necessary in carnivora, which 

 have to catch moving prey; the more complex the movements of the 

 fore-limbs, the greater becomes the necessity for fixation of the eyes 

 to guide them. In such animals each visual area corresponds with 

 the same half of both retinae, that is, with the opposite half of the 

 visual field ; the lower half of each area corresponds with the upper 

 half of each half field of vision, and vice versd. The appearance of 

 the macula lutea (with cortical representation in both hemispheres) 

 in the primates is the culminating point in visual development among 

 the mammals. 



