No. 3-] ACUTE VISION IN VERTEBRATES. 451 



animals was immersed in the Perenyi for from three to six 

 hours, when the anterior part of the eye was hardened so that 

 the cornea, lens, and vitreous humor were easily removed, 

 leaving the posterior half in situ. Better results are obtained 

 when the skin is removed from the head before immersion. 



Fig. 3. — Tern (Sterna hirundo) i/i. 



N. Nerve entrance. a. Band-like area. ft. Area and fovea temporalis. 



P. Pecten. fn. Area and fovea nasalis. 



With birds I have had good results, the retina lying back 

 smoothly so that the fovea and entrance of the optic nerve, 

 marked by the pecten, may be easily seen. Figs. 2 and 3 

 represent the appearance of the retina after the front of the 

 eye has been removed. With other animals, especially 

 mammals, there is a greater tendency for the retina to 

 wrinkle. 



Permanent demonstration material may^ioe prepared by 

 subjecting the whole head to the different fluids as described 

 for the hardening of the eye. It is not necessary, however, to 

 carry it farther than 80/0 alcohol when the front half of the 

 eye and vitreous humor may be removed. Such material is 

 preserved in 80/0 alcohol. 



Sections were made through the whole head of several 

 animals (fishes, amphibians, reptiles, birds, and some small 

 mammals) in order to determine approximately the angles 

 which the lines of vision make with the median plane. The 

 plane of section passed through each fovea or center of area 

 centralis and the center of the pupil. Fig. 4 represents such 

 a section through the foveae,/,/, of a chickadee's head (Parus 

 atricapillus), while the lines GH and GI show the axes of 

 vision. The axes of vision, owing to the mobility of the eye, 



