50 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



extent inter stitially, as is evidenced by the fact that the lacunae are rather 

 further apart in full-formed than in young bone. 



It will be seen that the common terms ossification in cartilage and ossifi- 

 cation in membrane are apt to mislead, since they seem to imply two processes 

 radically distinct. The process of ossification, however, is in all cases one 

 and the same, all true bony tissue being formed from membrane, perichon- 

 drium or periosteum; but in the development of such a bone as the femur, 

 lime salts are first of all deposited in the cartilage; this calcined cartilage, 

 however, is gradually and entirely reabsorbed, replaced by bone formed 

 from the periosteum. Thus calcification of the cartilaginous matrix pre- 

 cedes the real formation of bone. We must, therefore, clearly distinguish 

 between calcification and ossification. The former is simply the infiltration 

 of an animal tissue with lime salts, while ossification is the formation of 

 true bone. 



Growth of Bone. Bones increase in length by the advance of the 

 process of ossification into the cartilage intermediate between the diaphysis 

 and epiphysis. The increise in length indeed is due entirely to growth 

 at the two ends cf the shjjt. Increase in thickness in the shaft of a long 

 bone occurs by the deposition of successive layers beneath the periosteum. 

 If a thin metal plate be inserted beneath the "periosteum of a growing bone 

 it will soon be covered by osseous deposit, but if it be put between the fibrous 

 and osteogenetic layers it will never become enveloped in bone, for all the 

 bone is formed beneath the latter. 



THE TEETH. 



During the course of his life, man, in common with most other mammals, 

 is provided with two sets of teeth; the first set, called the temporary or milk- 

 teeth of infancy, are shed and replaced by the second or permanent set. 



Temporary Teeth. 



MIDDLE LINE OF JAW. 



Molars. 



2 



Canine. 



I 



Incisors. 

 2 



Incisors. 

 2 



Canine. 



I 



Molars. 



2 = IO 



The figures indicate in months the age at which each tooth appears : 



