270 



ADAPTATIONS TO SPACE AND MOTION 



intra-ocular muscles were known only in the birds, we would be ready to 

 argue that they had been developed for rapidity of action, without wait- 

 ing for experimental proof — for common sense would tell us that the 

 fast-flying birds must need extra-rapid accommodation. The situation in 

 the birds' only vertebrate competitors, the swift fruit-bats, seems corrobo- 

 rative; for it is easy to say that the reason why the Macrochiroptera have 

 given up all efforts to accommodate, and have produced the substitutive 

 retinal deformation described above, is because the early mammals inheri- 

 ted only slow-acting, smooth intra-ocular muscles from their particular 

 reptilian ancestors (see Fig. 60, p. 135) — or else had returned their 

 ciliary muscles to an unstriated condition before the bats evolved. 

 But what need have the plodding reptiles for any ultra-rapid accom- 



Fig. 107 — Scleral ossicles in sauropsidans. After Edinger. 



a, ossicular ring of Sphenodon punctatus, with '+' and '-' ossicles designated, xl'/i. 



b, skull of an eagle, Aquila chrysa'etos, with ossicular ring in place, x %. c, single ossicle 

 of A. chrysa'etos. x 1. d, eyeball of albatross, Diomedea regia, showing how ossicles sup- 

 port the concavity of the corneoscleral junction, x % . b- bursalis muscle; c- cornea; er- 

 external rectus; n- optic nerve; or- ossicular ring; so- superior oblique; sr- superior rectus. 



modatory capacity? Many lizards, of course, are remarkably agile — and 

 of all land reptiles the lizards have the most extensive and rapid accom- 

 modation. But this is most marked in the chameleon, than which no ver- 

 tebrate (unless it be the sloth) moves more slowly — except for its light- 

 ning-like tongue. The answer is that these and other lizards are insectivor- 

 ous: they need rapid accommodation as much because of the speed of 

 their prey as because of their own rapidity of movement. And even a 

 turtle has been seen to strike and grasp a grasshopper in flight. 



Scleral Ossicles in Sauropsida — The sauropsidan sclera typically 

 consists largely of a cartilaginous cup whose open rim extends quite close 

 to the limbus of the cornea. Just as typically the remaining zone of the 

 sclera is occupied by a circlet of thin overlapping plates of bone, the 

 scleral ossicles (Fig. 107). These are lacking only in crocodilians and 



