312 JAMES ROLLIN SLONAKER 



Chievitz ('87) says that in the human fetus the fovea begins 

 to form after the sixth month and is completed at seven and one- 

 half or eight months. In this process he finds that there is a 

 thinning of the ganghon-cell layer from seven cells deep to a 

 single row of cells in the center of the adult fovea. He further 

 says that all the retinal layers reach the adult condition two and 

 one-half months before birth. If he means by this that the fovea 

 is completely formed at this age, there is a discrepancy in his 

 statements. 



Five or six days after hatching the special region of the chorioid 

 shows a great mass of blood-vessels (figs. 95, 96, 97, and 103). 

 The center of the fovea is shown in figure 95. All the layers of 

 the retina are present, but they show a slight decrease in thickness 

 in the immediate center. 



B}^ the tenth day the pitting has become more pronounced 

 and the thickness of the chorioid much augmented. The inner 

 nuclear layer and the molecular layers show the greatest thinning. 



From this age on to the adult condition, as the fovea becomes 

 more perfect, the special thickening of the chorioid is reduced 

 and practically disappears with the complete formation of the 

 fovea. The chorioid, however, like the different layers of the 

 retina, is much thicker in the region of the optic axis than at 

 the ora serrata. The maximum thickness at the fovea is 0.134 

 mm. and the minimum at the ora serrata is 0.016 mm. Also, 

 the reduction in thickness is gradual, and the special area at 

 the earlier ages immediately back of the fovea has entirely 

 disappeared. On the temporal side of the fovea the chorioid 

 maintains almost a maximum thickness for about half the dis- 

 tance to the ora serrata. This is not trae of the nasal side. The 

 physiological reason for this, in my opinion, is the fact that this 

 portion of the retina is used more by the bird than the part on 

 the nasal side of the fovea. Owing to the lateral position of the 

 eyes in the head, the rays of light from the fields of vision in 

 front of it would necessarily stimulate the temporal portion of 

 the retina more than the nasal side. Since the existence of the 

 bird depends more on jDerception of the visual fields in front of 

 it and in the line of the optical axis than in any other direction, 



