480 OF NUTRITION. 



formed, however, it is not permeated by blood-vessels ; these being confined 

 to the capsule. During the early part of foetal life, and in inflammatory con- 

 ditions of this membrane, both the anterior and posterior portions of the Capsule 

 are distinctly vascular ; but at a later period, according to Mr. Toynbee, the 

 posterior half only of the capsule has vessels distributed over its surface ; and 

 these are derived from the Arteria Centralis Retinae. From optical experi- 

 ments which have been suggested to him by this circumstance, he infers that 

 " objects (radiating lines for instance), situated on the anterior surface of the 

 crystalline lens, produce an indistinctness in the image which is formed upon 

 the retina ; whereas, when these lines exist upon the posterior surface of the 

 lens, the image is clear," The substance of the Lens contains about 42 per 

 cent, of animal matter, with 58 parts of water. Nearly the whole of the former 

 may be dissolved in cold water by trituration ; the solution is coagulated by 

 heat, and forms a granular but not coherent mass ; alcohol and acids produce 

 the same effect. Hence it appears that the Lens chiefly consists of albumen 

 in its soluble form ; and this may be supposed to be contained in the cavities 

 of the cells, as it is in those of the vitreous humour. From the latest analyses, 

 it appears that the substance of the lens corresponds most with that modifi- 

 cation of albumen which forms the Globulin of the blood ( 573). In the 

 Vitreous humour we have an example of a very loose form of cellular tissue, 

 strongly resembling that which constitutes the entire structure of Acalephas 

 (Jelly-fish). That the Cells composing it have no open communication with 

 each other, is evident from the .fact that, when the general enveloping mem- 

 brane is punctured in several places, it is long before the contained fluid 

 entirely drains away. This fluid is analogous to that of the Aqueous humour ; 

 being little else than Water, holding a small quantity of Albumen and Saline 

 water in solution. From Mr. Toynbee's inquiries, it would appear, that the 

 vessels which pass through the Vitreous humour do not send branches into its 

 substance ; but that it is nourished by the vessels which are minutely dis- 

 tributed upon its general envelop. The Ciliary processes of the Choroid 

 membrane are almost entirely composed of large, plexiform vessels, which 

 allow a great quantity of blood to circulate through them ; and these have 

 probably an important share in the nutrition of the Vitreous body. 



628. In all the tissues hitherto described, the structure retains, more or less 

 completely, its original Cellular character ; and it is nourished by fluid supplied 

 to it, not by vessels permeating its own substance, but by those of the nearest 

 vascular part, with which, therefore, its surface only can come into relation. 

 In some of these tissues, especially the Epidermic, the parts once formed 

 would seem to undergo little or no subsequent vital change, but gradually lose 

 their connection with the living structure, and are at last thrown off and re- 

 placed by newly-formed parts. In these, a constant reproduction is taking 

 place ; and such tissues, when accidentally destroyed, are rapidly regenerated. 

 This is readily understood, when their very close proximity with an extremely 

 vascular membrane is considered ; the exudation from it speedily takes the 

 form of a layer of cells, and new ones are produced below, until the whole 

 thickness of the Cuticle or Epithelium is renewed. The Epidermoid tissues 

 are placed, by their peculiar functions, in the very circumstances which render 

 such a regenerating power necessary : in Cartilage, on the other hand, it is 

 not required, and does not exist. The functions of Cartilage are purely 

 mechanical ; the consolidation of its texture by internal deposit renders it little 

 disposed to change by spontaneous decay ; and it is protected by its toughness 

 and elasticity from those injuries to which softer or more brittle tissues are 

 liable. The^fe very circumstances, however, interfere with the activity of its 

 nutrition. Cells which are choked up with interior deposit do not readily 

 transmit fluid : it is doubtful whether any interstitial change can take place in 



