CARTILAGE AND CONNECTIVE TISSUE. 105 



and the intervertebral disks are also made up of this tissue. 

 Both elastic cartilage and fibro-cartilage often shade off 

 insensibly into pure elastic or pure white fibrous connective 

 tissue. 



Homologies of the Supporting Tissues. Bone, cartilage, 

 and connective tissue all agree in broad structural characters, 

 and in the uses to which they are applied in the Body. In 

 each of them the cells which have built up the tissue, with 

 few 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 various 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, therefore, one can 

 draw no hard and fast line between these tissues. 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 carti- 

 lages become hardened by the deposit 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 represented by cells only : these form 

 an intercellular 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 depending upon the mechanical properties 

 of their intercellular portions. 



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 connective 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 



