ACCOMMODATION IN BIRDS 279 



general processes described above are what actually take place. Hess 

 proved that there is no increased pressure on the vitreous, by showing 

 that in an excised eye the process will occur quite normally under elec- 

 trical stimulation, even though the posterior half of the eyeball be cut 

 away. 



Even though the lens does not move forward very far, it bulges for- 

 ward and encroaches upon the anterior chamber. The birds (except the 

 nocturnal ones) have left patent that same meridional, ventral slit in the 

 anterior uvea which we remarked in the elasmobranchs, to permit the 

 equalization of anterior and posterior chamber pressures. Despite this 

 provision, the aqueous pressure rises a little as noted above. In lizards 

 and turtles, the transversalis muscle runs through an homologous 

 aperture (Fig. Ill; see Fig. 110, /). 



The details of the accommodatory process in the crocodilians (which 

 have no scleral ossicles) remain to be worked out. The group has had 

 less attention than others, perhaps because material is hard to obtain in 

 Europe. Our abundant alligator is going begging for want of a curious 

 American physiologist. All that is known is that it does accommodate, 

 though very slightly and slowly. The beast is emmetropic, or a diopter 

 or so hypermetropic, in air. Under water, it must be 15-20 diopters of 

 more hypermetropic. 



Special Features in Birds and Lizards — The accommodating equip- 

 ment of birds differs from that of reptiles only in minor respects. The 

 transversalis muscle (never, apparently, concerned with accommodation 

 so much as with binocular vision) has been found only in the pigeon. It 

 may not be an homologue of the reptilian one, but rather an aberrant 

 slip of the ciliary muscle itself. 



The birds share with the lizards one muscle, Crampton's, which is 

 unique but is clearly a derivative of the ordinary reptilian ciliary muscle. 

 The reptilian ciliary, which is a husky descendant of the little ichthyop- 

 sidan tensor chorioidese, runs from the corneal margin and the inner sur- 

 face of the anterior sclera to the base-plate of the orbiculus ciliaris. The 

 situation in the birds is as if this reptilian ciliary muscle had been cut in 

 two all the way around the eye, half-way back along its course, the two 

 halves then being stretched enough to let the cut ends overlap (Fig. 112). 

 Crampton's muscle represents the corneal end of the reptile ciliary. Near 

 its posterior, inner surface is seen the insertion of a second (Briicke's) 

 muscle which continues backward toward the ora terminalis. Briicke's 



