OP NON-VASCULAR ANLMAL TISSUES. 179 



vitreous humour is semifluid. In the cornea one set of blood-vessels is prolonged 

 upon a part of its free surface; while another is in apposition with the margin 

 which is attached to the sclerotic, and is devoted to the nutrition of the principal 

 part of its substance. In having two sets of blood-vessels, the one devoted to the 

 nutrition of the surface of the tissue, and the other to that of its substance, the 

 cornea has an analogy with cartilage and fibro-cartilage. The crystalline lens re- 

 ceives a supply of nutrient fluid only at its surface, and by means of the ramifica- 

 tions of the arteria centralis retinae, which are distributed in the capsule which 

 surrounds it. The vitreous humour appears to be nourished by the fluid which it 

 derives from the vessels of the ciliary processes, the latter being received into sulci 

 of the humour. It will be observed, that the vitreous humour has vessels in contact 

 with it only at its surface, and at a small portion only of the latter ; it may be inferred 

 that the fluid brought to this part is capable of nourishing the whole vitreous body, 

 on account of the facility which its semifluid character allows of the diffusion of a 

 fluid through its entire mass. 



1. Of the Cornea. 



Structure of the Cornea. — It has been stated, in the Introduction to this paper, that 

 all the non-vascular animal tissues contain the characteristic corpuscles. The cornea 

 does not present any exception to this assertion, although the following account by 

 M. MiJLLER does not appear confirmatory of the accuracy of my opinion : the follow- 

 ing are his words: — 



"The middle layer, which constitutes the chief substance of the cornea, is formed 

 of an interlacement of bundles of bright fibres, without any intermixture of cor- 

 puscles*." 



I have frequently made most careful examinations of the substance of the cornea, 

 and I have always found corpuscles present. They are certainly not so abundant 

 here as in some of the above-named tissues. They are better seen in sections made 

 at right angles to the surface of the cornea, than in those parallel with its surface, 

 and they appear to be more evident after the cornea has been immersed in spirits 

 of wine. Some of these cells are rounded, others are oval, and have fine branches 

 radiating from them, similar to the osseous and pigment corpuscles. These cells 

 are surrounded by the bright fibres of which Muller has spoken ; these fibres, 

 which compose the larger portion of the cornea, are laxly connected together, so 

 as to have some analogy with cellular tissue ; the substance of the cornea being of 

 a loose texture, and easily pervaded by fluids. I believe that the lax texture of the 

 cornea allows of an easy penetration of its substance by the nutrient fluid which cir- , 

 culates around it, and thus there is not an equal necessity for the presence of the 

 corpuscles in this, as in the more dense tissues. 



* Elements of Physiology, by Baly. Appendix, p. 3. 

 MDCCCXLI. 2 B 



