SEC. 3. ON THE STRUCTURE OF THE INVESTMENTS 



OF THE RETINA. 



712. The Sclerotic Coat. This coat is thickest at the hind 

 pole of the eye, where it is pierced by the optic nerve at the 

 lamina cribrosa, and becomes gradually thinner forwards, though 

 at the extreme front, close to its junction with the cornea, it is 

 again somewhat thickened by the insertion of the ocular muscles. 

 It consists of bundles of ordinary white fibrillated connective-tissue, 

 with which are mingled, especially on the inner surface next to the 

 choroid, elastic fibres and connective-tissue corpuscles, the latter 

 frequently carrying pigment. The bundles run for the most part 

 in two directions, longitudinally, that is meridionally, and trans- 

 versely, that is equatorially, the two sets of bundles crossing each 

 other more or less regularly at right angles. The coat as a whole 

 therefore presents itself as a thin but tough investment, with some 

 but not very much extensibility and elasticity. 



It is somewhat scantily supplied with blood vessels, and though 

 it contains a large number of small lymph-spaces, lined with epithe- 

 lioid plates, possesses no proper lymphatic vessels. On its inside 

 next to the choroid is a large lymphatic space, the perichoroidal 

 space, with which the above smaller spaces communicate ; and a 

 similar large space exists on the outer surface between the sclerotic 

 itself and an investment of looser connective-tissue called Tenon's 

 capsule ; but we shall return to these when we come to speak of 

 the lymphatics of the whole eye. 



713. The Choroid Coat. This, which around the optic nerye 

 behind and at the junction of the cornea and sclerotic in front, 

 is closely attached to the sclerotic but loosely connected with it 

 elsewhere, consists essentially of blood vessels and of certain 

 muscular and nervous elements imbedded in connective-tissue 

 possessing special features. It is, in the first instance, a vascular 

 coat destined to nourish the all important retina ; but muscular 

 fibres, placed in certain parts of it, enable it to serve as a muscular 

 mechanism as well. 



The connective tissue, which forms the groundwork of the 

 coat, has the special feature of being almost entirely destitute of 

 white fibrillated bundles, at least in the choroid strictly so called. 



