60 THE CONNECTIVE TISSUES [CH. V. 



This periosteum, when the tooth is fully formed, is not only a 

 means of attachment of the tooth to its socket, but also in conjunction 

 with the pulp is a source of nourishment to it. Additional laminae of 

 cement are added to the root from time to time during the life of 

 the tooth (as is especially well seen in the abnormal condition called 

 an exostosis), by the process of ossification taking place in the perios- 

 teum. On the other hand, absorption of the root (such as occurs 

 when the milk-teeth are shed) is due to the action of the osteoclasts 

 of the same membrane. 



In this manner the first set, or milk-teeth, are formed ; and each 

 tooth, as it grows, presses at length on the wall of the sac enclosing 

 it, and, causing its absorption, is cut, to use a familiar phrase. 



The temporary or milk teeth are later replaced by the growth of 

 the permanent teeth, which push their way up from beneath them. 



Each temporary tooth is replaced by a tooth of the permanent set 

 which is developed from a small sac which was originally an offshoot 

 from the sac of the temporary tooth which precedes it, and called the 

 cavity of reserve (fig. 71, fp). Thus the temporary incisors and 

 canines are succeeded by the corresponding permanent ones, the 

 temporary first molar by the first bicuspid; the temporary second 

 molar develops two offshoots, one for the second bicuspid, the other 

 for the permanent first molar. The permanent second molar is budded 

 off from the first permanent molar, and the wisdom from the perma- 

 nent second molar. 



The development of the temporary teeth commences about the 

 sixth week of intra-uterine life, after the laying down of the bony 

 structure of the jaws. Their permanent successors begin to form 

 about ten weeks later. The second permanent molars originate about 

 the third month after birth, and the wisdom teeth about the third year. 



The Blood. 



A full consideration of the blood will come later, so here we shall 

 only rapidly and briefly enumerate its principal characters. 



The blood is a fluid which holds in suspension large numbers of 

 solid particles which are called the corpuscles. The fluid itself is 

 called the plasma or liquor sanguinis. It is a richly albuminous fluid ; 

 and one of the proteins in it is called fibrinogen. 



After blood is shed it rapidly becomes viscous, and then sets into 

 a jelly. The jelly contracts and squeezes out of the clot a straw- 

 coloured fluid called serum, in which the shrunken clot then floats. 



The formation of threads of a solid protein called fibrin from the 

 soluble protein called fibrinogen is the essential act of coagulation ; 

 this, with the corpuscles it entangles, forms the clot. Serum is plasma 

 minus the fibrin which it yields. The following scheme shows the 

 relationships of these constituents at a glance : 



