268 PHYSIOLOGY CHAP. 



resembles a spider's web, is subdivided into a posterior nervous 

 part, the retina proper, and an anterior epithelial part, the pars 

 ciliaris and pars iridica retinae (Fig. 106). 



The outer coat serves in virtue of its fibrous character as the 

 skeleton of the eyeball. In fact the motor muscles of the eye are 

 attached to the outer surface of the sclera as the skeletal muscles 

 are to the surface of the bones. In many animals the sclerotic 

 lias a tendency to ossify (birds, amphibia); the sclera of fishes 

 contains large plates of cartilage. The curvature of the cornea 

 forms the segment of a sphere of much shorter radius than that, 

 of the sclerotic. By analogy with the terrestrial sphere we 

 speak of an axis, a meridian, and an equator in the eyeball; the 

 line x i/ of Fig. 106 is the geometrical axis of the eye, joining the 

 anterior pole (mid-point of the cornea) with the posterior pole 

 (point on the sclerotic lyim: >omewhat externally to the entrance 

 of the optic nerve). The line a b, joining two points of the 

 maximal transverse diameter of the eye, passes through the 

 equator of the eyeball. The line ,/ i/ is about 23'5 mm. long in 

 the adult eye, the line // // about -4'',\ mm. long. 



The anterior /one of the sclerotic, from about the line of 

 attachment of the tendons of the redi muscles to the extreme 

 edge of the cornea, is covered by the conjunctiva, which is 

 reflected from the lids on to the eyeball, and is connected with 

 the sclerotic, by loose sub-con juuctival tissue. 



The uvea is also known as the tunica vasculosa, from the 

 great number of blood-vessels, united by connective tissue, of 

 which it is made up. Its posterior portion or choroid coat and 

 its middle or ciliary portion are applied to the inner surface of 

 the sclerotic, with which they are united by means of a few 

 filaments of connective tissue, vessels and nerves, which leave a 

 small lymph space between the two coats, and enable the choroid 

 to move on the sclerotic. The anterior portion of the uvea, the 

 iris, is not in contact with the sclerotic, and the space between 

 it and the cornea is known as the anterior chamber of the eye. 

 Posteriorly the choroid is pierced by the optic nerve; and the 

 centre of the iris is perforated by a circular aperture, the pupil. 

 At the junction of the sclerotic and the cornea the uvea is firmly 

 attached to the sclerotic by the ligarnenturn pectinatum. 



The choroid is a brown membrane (black in most animals), 

 which is soft, extensible, and easily lacerated. Three layers can 

 be artificially distinguished in it : the external or lamina fusca, 

 so called from the number of spindle-shaped or branched and 

 more or less thickly pigmented connective-tissue cells that are 

 scattered in it ; the middle and thicker layer, consisting mainly 

 of blood-vessels, but containing also a number of pigment cells, 

 scattered in the interstitial spaces ; the internal layer, represented 

 by a hyaline membrane, which is transparent and elastic, and 



