76 THE CONNECTIVE TISSUES [CH. V. 



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 aspacts of physiology, but it will be impos- 

 sible to discuss all the other phenomena we shall have to study in 

 the meanwhile 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 : 



f Serum 



-P, , f Plasma I Fibrin ) nl , 



Blood I 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 different 

 groups of the vertebrates. In mammals they are biconcave (except 

 in the camel tribe, where they are biconvex) non-nucleated discs, in 

 man -rsW i- ncn i n diameter ; during foetal life nucleated red corpuscles 

 are, however, found. In birds, reptiles, amphibians and fishes they 

 are biconvex oval discs with a nucleus: they are largest in the 

 amphibia. The most important and abundant of the constituents 

 of the red corpuscles is the pigment which is called haemoglobin. 

 This is a proteid-like substance, but is remarkable as it contains a 

 small amount of iron (about 0'4 per cent.). 



The blood during life is in constant movement. It leaves the 



