248 ANATOMY OF VERTEBRATES. 



In the Ornithorhynchus the eyeball is small and spherical; 

 the sclerotic fibre-cartilaginous, the cornea flabby, the retina 

 thick : there is no trace of pecten or marsupium : the lens is two 

 lines in transverse diameter, one line in antero-posterior diameter ; 

 the anterior surface is nearly flat, the posterior very convex. 

 The choroid is black, without a tapetum lucidum ; the pupil is 

 circular. 



The anatomy of the eye offers no peculiarity illustrative of the 

 affinities of the Marsupialia or of any other speciality in their 

 economy save the nocturnal habits of the majority of the order. 

 It is in relation to these habits that the lens is large and convex, 

 the iris broad, the pupil round and very dilatable, and the cornea 

 correspondingly large. The eye is relatively large in the swift- 

 moving, far-ranging Kangaroos : I found the dark pigment on 

 both the inside and outside of the choroid ; the ciliary processes 

 are long: the lens is proportionally large. In the dead Kan- 

 garoo the radiated muscle of the iris is much contracted, and the 



o 



pupil widely open. The eye is small in Didelplds virginiana ; 

 the pupil is round : the lens very convex. 



The Insectivora have small eyes : the moles least of all. In a 

 great pipe-toothed shrew (Solenodon) one foot in length, exclusive 

 of tail, the palpebral opening does not exceed three lines, and 

 there is no distinction between orbit and temporal fossa. Bats 

 have the smallest eyes of all volant Vertebrates. In Rodents 

 the size of the eyeball bears relation to the extent and swiftness 

 of locomotion, and is greatest in Jerboidce and Leporidce. The 

 position of the eyes is always lateral, and by the prominence of 

 the cornea they are susceptible in these timid quadrupeds of re- 

 ceiving the image of a pursuer. In the hare and other rodents 

 the retina seems to expand from the divisions of a cleft termina- 

 tion of the optic nerve, within the eyeball. The pupil is round 

 in most Rodents : in a dead Agouti it was a horizontal ellipse. 

 In the squirrel the ante-retral diameter of the eyeball is to the 

 transverse as 11 to 12 : in the hare it is as 23 to 25. J In all 

 the order Bruta the eyes are relatively small : in the sloths the 

 contracted pupil is a vertical slit. 



In Cetacea the eyes are small, especially in relation to the 

 bulk of the larger kinds : and the essential part of the organ is 

 still less, owing to the thickness of the sclerotic, fig. 195, a, a, 

 and this increases from the cornea, b } backward to the long, 



1 A table of these dimensions of the eye in different Vertebrates will be found in 

 xii. iii. p. 390 ; also in cvi". 



