THE ORGANS OF SENSE. 115 



cells, resting on an elastic, homogeneous membrane, called the posterior 

 elastic lamina. In the inner part of the sclera, close to its junction 

 with the cornea, is a vein, the canal of Schlemm (sinus venosus sclerce), 

 which encircles this part of the eyeball. Behind the sinus is a pro- 

 jecting ridge of the sclera, the scleral spur, which forms the point of 

 origin of the radial fibres of the ciliary muscle. At the margin of the 

 cornea the posterior lamina breaks up into trabeculse, some of which 

 are attached to the anterior surface of the scleral spur, while the others 

 form the ligamentum pectinatum iridis, which is continuous with the 

 substance of the iris. The angle between cornea and iris is known as 

 the nitration angle, because here the aqueous humor drains between 

 the fibres of the ligamentum pectinatum iridis into the canal of 

 Schlemm. 



The middle coat consists of the choroid, ciliary processes, and iris, 

 together forming the vascular tunic of the eyeball. The choroid is 

 separated from the sclera by a lymph space, which, however, is traversed 

 by strands of non-vascular fibrous tissue, constituting the lamina 

 suprachoroidea, and forming the outer layer of the choroid itself. 

 Internal to the lamina suprachoroidea is a layer containing the larger 

 blood-vessels, and internal to that is another layer, the chorio-capillaris, 

 which contains a network of capillaries ; the capillary layer is bounded 

 internally by a structureless membrane, the lamina basalis. Scattered 

 throughout the choroidal tissue are numerous pigment cells. 



The choroid lines the sclera to within a short distance of the sclero- 

 corneal junction, and is continued forward from that point as the ciliary 

 processes, about seventy in number, which from behind appear as a circle 

 of radially arranged vascular projections. The posterior surface of the 

 ciliary processes is covered by a double layer of cubical pigment cells, 

 forming the forward prolongation of the retina, and called the pars 

 ciliaris retinae,. The substance of the ciliary processes consists of 

 connective tissue with pigment cells and blood-vessels, together with 

 the ciliary muscle. The latter is composed of smooth muscle fibres, and 

 consists of a radial and a circular portion. The radial fibres arise from 

 the scleral spur and pass backwards to be inserted into the ciliary 

 processes and choroid ; the circular fibres form a bundle which lies 

 internally to the radial portion. 



The iris is continued forward from the ciliary processes and is incom- 

 plete in front, leaving a circular aperture, the pupil. The iris is 

 composed of connective tissue, with a variable number of pigment cells 

 and numerous blood-vessels. In dark eyes the pigment cells are 

 numerous ; in blue eyes they are fewer in number. The fibres of the 

 ligamentum pectinatum iridis connect the iris with the posterior lamina 



