158 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



Fig. 56. fibres, laxly connected together, so as to re- 



semble areolar tissue. Two sets of vessels, a 

 superficial and a deep-seated, surround the mar- 

 gin of the cornea. The arteries of the former 

 are prolonged for a short distance upon the 

 Conjunctival membrane, which forms the outer 

 lamina of the cornea ; but they terminate in 

 veins at from 5 to 5 a line from its margin. 

 The deep-seated vessels belong to the Cornea 

 proper ; but they do not enter it, the arteries 

 terminating in veins just where the tissue of 

 the Sclerotic becomes continuous with that of 

 the Cornea. In diseased conditions of the 

 Cornea (as of the articular cartilages), both sets 

 of vessels extend themselves through it ; the 

 superficial not unfrequently form a dark band 

 of considerable breadth round its margin ; 

 whilst the deep-seated are prolonged into its 

 entire substance. Notwithstanding the absence 

 of vessels in the healthy condition of this struc- 

 ture, incised wounds commonly heal very rea- 

 dily, as is well seen after the operation of. ex- 

 traction of Cataract ; but the foregoing details 

 make evident the importance of not carrying 



the incision further round than is necessary ; since the corneal tissue should 

 not be cut off from the supply of nourishment, afforded by the vessels in its 

 immediate proximity. 



[This structure has been recently studied by Messrs. Todd and Bowman, and is described 

 by them with great accuracy. We subjoin their description. " The cornea, though a beau- 

 tifully transparent substance, and appearing at first sight as homogeneous as glass, is never- 

 theless full of elaborate structure. It is in fact composed of five coats or layers, clearly 

 distinguishable from one another. These are, from before backwards, the conjunctival layer 

 of epithelium, the anterior elastic lamina, the cornea proper, the posterior elastic lamina, and the 

 epithelium of the aqueous humour, or posterior epithelium. The cornea, when uninflamed, con- 

 tains no blood-vessels; those of the surrounding parts running back in loops, as they arrive 

 at its border. 



On the cornea proper, or lamellated cornea, the thickness and strength of the cornea mainly 

 depend. It is a peculiar modification of the white fibrous tissue, continuous with that of the 

 sclerotic. At their line of junction (fig. 57), the fibres, which in the sclerotic have been 



Fig. 57. 



ft 



Nutrient Vessels of the cornea. A, 

 superficial vessels belonging to the 

 Conjunctival membrane, and continu- 

 ed over the margin of the Cornea; B, 

 vessels of the Sclerotic, returning at 

 the margin of the Cornea. 



Vertical section of the Sclerotic and Cornea, showing the continuity of their tissue between the dotted 

 lines: a. Cornea, b. Sclerotic. In the cornea the tubular spaces are seen cut through, and in the 

 sclerotic the irregular areolfo. Cell-nuclei, as at c. are seen scattered throughout, rendered more plain 

 by acetic acid. Magnified 320 diameters. 



