THE SENSES 541 



the visuo-sensory area, of which something has been said at p. 500. 

 Mott has described the visual cortex of insectivora, rodents, marsu- 

 pials, ungulates, carnivora, lemurs, and primates, * and points out that 

 the more the animal depends on vision as a directive faculty in its 

 preservation, the more complex the structure of the visual cortex 

 becomes. For example, in the mole and shrew, who are probably 

 only able to perceive light from darkness, the cell structure of the 

 visual cortex consists almost entirely of granular-like small stellate 

 cells ; below this is a thin layer of polymorph cells, and there occa- 

 sional large pyramidal-shaped cells occur. In the hedgehog the cells 

 are more complex, but the type of cell- lamination is simple. 



In rodents the visual cortex is becoming more complex, and in the 

 rabbit there are large stellate and branching pyramidal cells, and 

 a well-marked line of Gennari. In ungulates the cell lamination 

 exhibits a relatively deep polymorph layer, numerous solitary cells 

 of Meynert, large and small stellate cells, a fair number of cells 

 corresponding to the pyramidal layer, and a well-marked line of 

 Gennari. In canince there is an extensive area of visual cortex, 

 and in addition to the layers common to the orders described, it 

 has a fair depth of pyramidals. Infelidce the striking feature is the 

 depth of the pyramidal layer, the solitary cells of Meynert are 

 numerous, and the polymorph layer relatively diminished. 



On to the visuo-sensory area the retinas are projected by means of 

 the occipito-thalamic fibres ; but the impulses received are not there 

 analysed. Conscious visual sensations, and especially visual asso- 

 ciations and memories, are developed in the visuo-psychic area, 

 and this, as Mott has shown, is distinguished by the presence of 

 many small and medium-sized pyramidal cells in its outer layers, 

 and the absence of the line of Gennari. The visuo-psychic area in 

 animals is structurally not nearly so distinct as in man. 



Watson, whose work on the structure of the cerebral cortex of 

 animals has been referred to at p. 502, considers that the visual 

 area in ungulates and carnivora has not the definite appearance 

 of a projection sphere ; he believes that in the lowest mammals 

 vision has not a very definite higher cortical representation.! 



In animals with binocular stereoscopic vision the right occipital 

 cortex receives the visual impression from the two right halves of 

 the retinas, the left occipital lobe impressions from the two left 

 halves of each retina. In animals where the eyes are laterally 

 situated and binocular vision impossible, the visuo-sensory area 

 receives a picture from the opposite eye, and from that only, so that 

 two different pictures are implanted at one and the same time on the 

 right and left cortical visuo-sensory areas of such animals. 



The Ophthalmoscope. — In order to examine the structures 

 posterior to the iris, a mirror with a hole in the centre is applied 

 to the eye of the observer, so that he can see through the hole 

 into the observed eye ; from a suitable source of light rays are 

 reflected by the mirror through the pupil on to the retina to be 



* ' The Progressive Evolution of the Structure and Functions of the 

 Visual Cortex in Mammalia.' Archives of Neurology, vol. hi., 1907. 

 F. W. Mott, M.D., F.R.S. 



f ' The Mammalian Cerebral Cortex, with Special Reference to its 

 Comparative Histology.' Archives of Neurology, vol. hi., 1907. G. A. 

 Watson, M.B., CM. 



