CHAPTER III. 

 FORM-SENSE. 



ACUITY of form-sense reaches its highest development 

 among mammals in the Primates. Mammals as a class are 

 characterized by the absence of a fovea, the Primates being 

 the only ones in which it is to be found. In many mammals 

 which do not possess a fovea there is an area centralis which 

 approaches in structure the yellow spot; this can be recog- 

 nized ophthalmoscopically, when the retinal bloodvessels 

 are present, by the absence of them in that region and by 

 their distribution around it. In sections this area centralis 

 is distinguishable by an increased thickness of the retina, 

 and microscopically by the arrangement of the cells. The 

 increased thickness is due to the greater length of the rods 

 and cones, their close congregation together, and the larger 

 number of cells in the nuclear and ganglion cell layers associ- 

 ated with them. 



Chievitz 19 and Slonaker 20 have examined the eyes of a 



v 



number of different species of mammals anatomically, and 

 the latter has published their combined findings in tabular 

 form. In some animals, such as rats and bats, no central 

 area is discoverable. In rabbits, most of the Ungulates 

 and the fox, there is a horizontal bandlike area, which in 

 the horse measures 5 to 7 mm. in breadth. Cats and some 

 other Carnivora have a rounded central area. Krause 21 

 stated that cats possessed a fovea, but this was not found 

 to be the case by Ganser 22 and Chievitz. 19 Lindsay Johnson 3 



