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CHAPTER IV. 



PASSIVE ORGANS OF LOCOMOTION, CONTINUED. OF CARTILAGE 



AND FIBRO-CARTILAGE. 



CARTILAGE is extensively used in the animal frame, and is one of 

 the simplest of the textures. Like the adipose tissue, it approaches 

 very closely in its intimate structure to the cellular tissue of 

 vegetables. 



In the development of the embryo, it is one of the first tissues 

 to appear as a distinct structure, and it constitutes the internal 

 skeleton in its earliest condition in the animal scale. The rudi- 

 mentary skeleton of the cephalopoda consists of it; and in one class 

 of fishes (hence termed cartilaginous, as the shark, ray, lamprey) 

 the skeleton is entirely composed of it. 



In man, and the higher animals, cartilage is employed tempora- 

 rily, as a nidus for bone, in the early stages of life, and is then 

 called temporary cartilage. This, at a certain period, begins to 

 ossify, and finally disappears by being converted into bone. At one 

 time, the greatest part, not the whole, of the skeleton is cartila- 

 ginous ; and for a considerable period after birth the extremities of 

 the long bones are chiefly composed of cartilage, and the larger pro- 

 cesses are connected to the shaft of the bone by this substance. 



For other purposes, however, a cartilage is employed which is not 

 prone to ossify, viz. permanent cartilage, and this is used either in 

 joints (articular cartilage), or in the walls of cavities (membrani- 

 form cartilage). The articular variety is either disposed as a thin 

 layer between two articular surfaces, and equally adherent to both, 

 as in the synarthrodial joints (the cranial sutures, the sacro-iliac 

 symphyses, &c.); or it forms an encrustation upon the articular 

 ends of the bones entering into the composition of diarthrodial 

 joints ; thus, the extremities of the femur, tibia, the arm-bones, &c. 

 are all coated with a layer of cartilage, moulded to the shape of the 

 articular surfaces. The membraniform cartilages are not employed 

 in connection with the locomotive mechanism, but serve to guard the 

 orifices of canals or passages, or to form tubes, that require to be 

 kept permanently open ; the elasticity of the material effecting this 



