14 ELEMENTARY BIOLOGY. [CHAP. 



lage which forms the core of the ramus of the lower-jaw or 

 mandible. 



The floor of the mouth is supported by a plate of carti- 

 lage (the hyoid), this is attached to the skull by a slender 

 cartilaginous band or cornu which is connected with the 

 auditory capsule. 



The pectoral and pelvic arches (see Laboratory work) 

 are, in the young state, paired undivided cartilages, and the 

 development of bone in and upon them does not really 

 destroy this continuity, the cartilage persisting at the ends 

 of the bones, and between them in the cavities for articu- 

 lation of the limb bones. 



In like manner, the bones of the limbs consist originally 

 of cartilaginous models of the perfect bone ; but, as deve- 

 lopment proceeds, the middle of the cartilaginous model 

 commonly becomes invested by a sheath of true bone, while 

 calcareous deposits take place in the cartilage close to its 

 growing extremities. As the bone grows, the superadded 

 sheath invades the middle of the cartilage and more or less 

 replaces it; while the terminal portions of cartilage continue 

 to grow and enlarge, and the calcareous deposit within them 

 increases, without however reaching their surfaces. Thus 

 one of the larger adult limb bones (say the femur) consists 

 of a median portion or shaft, and of two terminal caps of 

 cartilage containing calcified epiphyses overlying, and more 

 or less enclosed within, the hollow ends of the shaft. 



Seeing that the bony elements of the adult skeleton arise 

 by superaddition to and replacement of a cartilaginous pre- 

 decessor, it follows that those portions of it which remain 

 permanently cartilaginous are persistent representatives of 

 latter or embryonic endoskeleton. 



The general disposition of the parts which are seen in 

 the mouth has already been described. 



