82 THE CONNECTIVE TISSUES. [CH. v. 



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 the sixteenth week of intra-uterine life. 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, when we 

 know more about the chemical aspects of physiology, but it 

 will be impossible to discuss all the other phenomena we shall 

 have to study in the meanwhUe without some elementary 

 knowledge of the principal properties of this fluid. For 

 that reason, and also to complete our list of the connective 

 tissues, we may here 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 proteids 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 proteid called fibrin from 

 the soluble proteid we have called fibrinogen is the essential 

 act of coagulation; this, with the corpuscles it entangles, is called 

 the clot. Serum is plasma minus fibrin. The following scheme 

 shows the relationships of the constituents of the blood at a 

 glance : 



( Serum 



T,, , ( Plasma j Fibrin ) ~. , 



Blood \ , \ Clot. 



( Corpuscles ) 



The corpuscles are of two chief kinds, the red and the white. 

 The white corpuscles are typical animal cells, and we have 

 already made their acquaintance when speaking about amoeboid 

 movements. 



The red corpuscles are much more numerous than the white, 

 averaging in man 5,000,000 per cubic millimetre, or 400 to 500 

 red to each white corpuscle. It is these red corpuscles that give 

 the red colour to the blood. They vary in size and structure in 



