184 COMPARATIVE ANATOMY. 



of light pass. The amount of light admitted is regulated by the 

 dilator and constrictor (sphincter) muscles of the iris, which are 

 able to increase or lessen the size of the pupil ; the iris thus serves 

 as a screen to regulate the amount of light which enters the eye. 



Not only is the size of the pupil inconstant, but the lens is also 

 capable of undergoing considerable change in form, becoming more 

 flattened, or more convex, as the case may be. The former con- 

 dition occurs when distant, the latter when near objects are looked 

 at. This delicate accommodating apparatus is regulated by a 

 muscle (the ciliary, or tensor choroidese), which arises in a 

 circle all round the eye from the point of junction of the iris and 

 sclerotic, and is inserted along the peripheral border of the iris 

 (Fig. 152, Zc). 



External to the vascular layer of the choroid lies a lymph-sinus 

 (perichoroideal sinus) the walls of which are known under the 

 name of lamina fusca (Fig. 152, Lf), and finally, externally 

 to this is a firm, fibrous, partly' cartilaginous or even ossified 

 layer, the sclerotic (Fig. 152, Sc). While the latter passes inter- 

 nally into the sheath of the optic nerve (OS), and from thence 

 into the dura mater, it becomes continuous externally with the 

 cornea, the outer surface of which is covered over by an epithe- 

 lial layer continuous with the epidermis, the conjunctiva (Fig. 

 152, Co, Cf). The sclerotic and cornea together form a firm outer 

 support for the eye, and thus, together with the gelatinous mass 

 of the vitreous humour, guarantee the rigidity necessary for 

 the physiological activity of the nerve end -apparatus. Between 

 the cornea and iris there is a large lymph-space, the so-called 

 anterior chamber of the eye (Fig. 152, VK), its contained fluid 

 being called the aqueous humour. 



The deep orbit, formed by the skull, serves as a further pro- 

 tection for the eye, as do also certain accessory structures, which 

 may be divided into three categories, viz. : 



1. Eyelids (Palpebrae). 



2. Glandular organs. 



3. Muscles (apparatus for moving the eye-ball). 



The eyeball then is formed of a series of concentric layers 

 which are called from within outwards retina, choroid (with the 

 iris) (vascular layer), and sclerotic (with cornea) (skeletal layer). 

 The first corresponds with the nervous substance of the brain, the 

 oecond with the pia mater, and the third with the dura mater. 

 The interior of the eye contains refractive media, the lens and 

 vitreous humour. To these, certain accessory structures are 

 added. 



Fishes. 1 The eyo of Cyclostomes remains at a very low 

 stage of development : this is seen not only in the structure of 

 the retina, but also (that is, in Myxinoids) in the absence of the 



1 In Aniphioxus the presence of a visual organ has not been certainly proved. 



