38 NATURAL HISTORY OF VERTEBRATES. 



in position and alter the form of the lens, but also serve to regulate the admission of 

 lio-ht to the eye by modifying the size of the pupil (the iris). Light is admitted to 

 the eye through the cornea, a modified piece of skin, the mesoblastic layer of which 

 is continuous with the capsule of the eye, or sclerotic coat. Tiie latter is opaque, and 

 cartilaginous or occasionally ossified in plates. Unlike 

 the auditory capsule, it forms no integral part of the 

 skull, although in some sharks it is continuous by a car- 

 tilaginous stalk with the cranial capsule. 



The eyeball, composed of the elements just enu- 

 merated, is surrounded in the orbit by a quantity of 

 loose tissue which admits of free movement in differ- 

 ent directions, under the control of the muscles at- 

 tached to the sclerotic coat. These are generally six 

 in number, four of them straight and parallel to the 

 Fig. 41.— Diagram iif vertebrate eye; optic nerve, the remaining two oblique, and arising in 

 pVoTet'sri'/i'risforopticlervef'% ^^^^ anterior angle of the orbit. We have already dis- 

 noiJcBie'«)i Tftreous\"umo'r. '''^^'''"' cussed the origin and innervation of these muscles. 

 Protective folds of skin to cover the eyeball are de- 

 veloped in most forms above fishes, being rendered necessary by the change from 

 aquatic life. These folds are the upper and lower eyelids, with frequently a third, 

 which can be pulled over the eye from the anterior angle, and is called the ' nicti- 

 tatin<T membrane.' Tiiis is kept moist by the secretion of a special gland (Harde- 

 rian), while the secretion of the lachrymal gland, which is situ.ated at the posterior 

 angle of the orbit, is being kept in constant motion over the surface of the eyeball by 

 the movements of the eyelids proper. Between these and the surface of the eyeball 

 is a space, the conjunctival sac, which is put in communication with the nasal cavity 

 by the lachrymo-nasal duct, through which the tears are drained into the nasal cavity. 

 Before leaving the eye it is necessary to consider shortly certain structures of 

 restricted occurrence within the vertebrate series. Many bony fishes have an import- 

 ant vascular organ, the choroid gland, formed round the optic nerve after it has 

 pierced the sclerotic, the function of which is very obscure. It will be more con- 

 venient to refer later to the mode in wliich it is supplied with blood. Another struc- 

 ture, of the nature of a vascular plexus, is the pecten of reptiles and birds : it projects 

 into the vitreous humor, and may be considered to replace the absent vessel of the 

 retina in the forms which possess it. The obstruction which the blood meets with in 

 flowing through such a vascular plexus leads to the effusion of nutriti\'e material 

 through the thin walls of the vessels of the plexus. 



Immediately outside the retina, certain .sti-uctures .are developed within the choroid 

 coat, which appear to have the function of rendering that coat more impei'vious to 

 light. Such .are the ' tapeta' of many fishes and of some mammals. The focussing 

 of the lens is not .always secured by the ciliary part of the choroid coat, but there 

 may be a special muscular falciform process projecting into the vitreous humor, and 

 grasping the lens, as in the case of most bony fishes. 



Apart from the presence or absence of vessels referred to above, the retina may 

 exhibit other structural differences within the different groups. In the retina there is 

 a considerable quantity of indifferent isol.ating material, which serves for the support 

 of the really nervous elements, and which corresponds to the neuroglia of the brain. 

 The elements which are directly acted upon by the light are the rod-like or cone-like 



