XIII 



PHYLUM CHORDATA 



103 



the orbit, and covered externally by a thick coat of cartilage or of 

 dense fibrous tissue, the optic capsule or sclerotic (Fig. 739, scL). 



Tt 





A 



FIG. 737. A, vertical section of one of the papilla? of the tongue of a Mammal. <7, sub- 

 mucosa ; c. epithelium; n. nerve-fibres; t. taste-buds. B, two taste-buds, c. covering cells 

 shown in lower bud; d, sub-mucosa ;) e. epithelium of tongue; m, sensory processes; />, 

 internal sensory cells shown in upper bud. (From Fester and Shore's Physiology.) 



On the outer or exposed portion of the eye the sclerotic is replaced 



by a transparent membrane, the cornea (c.), formed of a peculiar 



variety of connective tissue, and covered on both its outer and 



inner faces by a layer of epithelium. The curvature of the cornea 



is not the same as that of the sclerotic, 



so that the whole external coat of the 



eye has the character of an opaque 



spherical case the sclerotic, having i 



circular hole cut in one side of it and 



fitted with a transparent window- -the 



cornea. The latter is almost flat in 



Fishes, but bulges outwards in terrestrial 



Vertebrates. 



Lining the sclerotic is the second 

 coat of the eye the choroid (ch.) formed 

 of connective tissue abundantly supplied 

 with blood vessels. At the junction of 

 sclerotic and cornea, it becomes continu- 

 ous with a circular membrane (7), placed 

 behind but at some distance from the 

 cornea and called the iris. This latter is 

 strongly pigmented, the colour of the 

 pigment varying greatly in different 

 species, and giving, as seen through the 

 transparent cornea, the characteristic 

 colour of the eye. The iris is perforated 



in the centre by a circular or slit-like aperture, the pupil, which, 

 in the entire eye, appears like a black spot in the middle of the 

 coloured portion. Except in Fishes, the pupil can be enlarged 

 by the action of a set of radiating unstriped muscle-fibres con- 



F IG . 738. Epithelial cells of 

 olfactory mucous membrane. 

 A, of 'Lamprey; B, of 

 Salamander. E, inter- 

 stitial cells ; R, olfactory 

 cells. (From Wiedersheim's 

 Vertebrata.) 



