166 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



devised for the purpose, that a moderate vascularity of the posterior 

 capsule does not interfere with distinct vision ; whilst if the anterior 

 capsule were traversed by vessels, the picture on the retina would be 

 no longer clear. The substance of which the lens is composed, ap- 

 pears to be soluble Albumen, or perhaps more closely resembles the 

 Globulin of the blood. 



276. The Vitreous body, which fills the greater part of the globe of 

 the eye, also seems to possess a cellular structure ; the cells containing 

 a fluid, which is little else than water holding in solution a small 

 quantity of albumen and saline matter ; and the membrane which forms 

 their walls being so pellucid as to be scarcely distinguishable. Indeed 

 the cellular character of this substance is chiefly inferred from the fact, 

 that when its capsule or enveloping membrane is punctured, even in 

 several places, the contained fluid does not speedily drain away, as 

 it would do if it were merely contained in the interstices of an areolar 

 tissue. The blood-vessels which traverse the Vitreous body do not 

 send branches into its substance ; and it must derive its nutriment from 

 those which are distributed minutely upon its general envelope, and 

 probably also from the large plexiform vessels of the ciliary processes 

 of the Choroid coat. 



277. Before proceeding to describe the structure of Bone, to which 

 it seems natural to pass on from Cartilage, it will be useful to advert to 

 the modes in which the tissues of Invertebrated animals are consolidated 

 by deposits of solid matter, in order that they may afford the requisite 

 support and protection, without that interstitial growth which is pecu- 

 liar to the skeletons of the Vertebrated classes. Commencing with the 

 Polypifera, or Coral-forming animals, we observe that their strong axes 

 or sheaths are destined only to give support to their softer structures, 

 and that the parts once consolidated undergo no subsequent change. It 

 was formerly imagined, that the stony Corals were "built up" by the 

 animals which form them, somewhat in the same manner as a Bee con- 

 structs its cell. But it is now fully demonstrated, that the calcareous 

 matter (which here consists solely of the Carbonate of Lime) is combined 

 with the living tissue ; and that the most solid mass of Coral thus has 

 an organized basis. The proportion of earthy to animal matter, how- 

 ever, is so great in these structures, that very little, if any, nutrient 

 changes can take place in their tissues, when once it has become con- 

 solidated. Such changes are not, however, required. The substance 

 thus developed by the attractive power of the soft gelatinous tissues, 

 which draw into themselves the small quantity of calcareous matter dis- 

 solved in the surrounding water, is so little disposed to undergo change, 

 that it will maintain its solidity for centuries ; and even when acted on 

 by water or by heat, it does not undergo disintegration, for its calcareous 

 particles arrange themselves in a new method, and become converted 

 into a solid crystalline rock. Such rocks, the product of the metamor- 

 phosis of ancient coral-formations, make up a large proportion of the 

 external crust of the earth. The solid stem or sheath, once con- 

 solidated, appears to undergo no further change in the living Coral- 

 structure ; for its increase takes place, not by interstitial but by super- 

 ficial deposit, that is, not by the diffusion of new matter through its 



