108 THE HUMAN BODY. 



with rare exceptions, form an inconspicuous part of it in 

 its fully developed state, the chief mass of it consisting 

 of intercellular substance. In hyaline cartilages this latter 

 is not fibrillated; but these cartilages pass insensibly in va- 

 rious regions of the Body into elastic or fibro-cartilages, and 

 these latter in turn into elastic or fibrous connective tissue. 

 The lamellae of bone, too, when peeled off a bone softened 

 in acid and examined with a very high magnifying power, 

 are seen to be pervaded by fine fibres. Structurally, there- 

 fore, one can draw no hard and fast line between these tis- 

 sues. The same is true of their chemical composition; bone 

 and white fibrous tissue contain a substance (collagen) 

 which is converted into gelatin when boiled in water; and 

 in old people many cartilages become hardened by the de- 

 posit in their matrix of the same lime salts which give its 

 hardness to bone. Further, the developmental history of 

 all of them is much alike. In very early life each is repre- 

 sented by cells only : these form an intermediate substance,, 

 and this subsequently may become fibrillated, or calcified,, 

 or both. Finally they all agree in manifesting in health no- 

 great physiological activity, their use in the Body depend- 

 ing upon the mechanical properties of their intercellular 

 substance. 



The close alliance of all three is further shown by the 

 frequency with which they replace one another. All the 

 bones and cartilages of the adult are at first represented 

 only by collections of connective tissue. Before or after 

 birth this is in some cases substituted by bone directly (as 

 in the case of the collar-bone and the bones on the roof of 

 the skull), while in other cases cartilage supplants the con- 

 nective tissue, to be afterwards in many places replaced by 

 bone, while elsewhere it remains throughout life. 



Moreover in different adult animals we often find the 

 same part bony in one, cartilaginous in a second, and com- 

 posed of connective tissue in a third: so that these tissues 

 not only represent one another at different stages in the 

 life of the same animal but permanently throughout the 

 whole life of different animals. Low in the animal scale 



