CHAP, i.] BLOOD. 33 



group, and other matters, among which is the peculiar complex 

 fat called lecithin, of which we shall have to speak in treating 

 of nervous tissue. In the nucleated red corpuscles of the lower 

 vertebrata this differentiated stroma, though forming the chief 

 part of the cell body around the nucleus, is accompanied by a 

 variable amount of undifferentiated protoplasm, but the latter in 

 the mammalian red corpuscle is either absent altogether or reduced 

 to a minimum. Whether any part of this stroma is living, in the 

 sense of being capable of carrying on a continual double chemical 

 change, of continually building itself up as it breaks down, is a 

 question too difficult to be discussed here. 



The red colouring matter which in normal conditions is asso- 

 ciated with this stroma may by appropriate means be isolated, and, 

 in the case of the blood of many animals, obtained in a crystalline 

 form. It is called Hcemoglobin, and may by proper methods be 

 split up into a proteid belonging to the globulin group, and into a 

 coloured pigment, containing iron, called Hcematin. Haemoglobin 

 is therefore a very complex body. It is found to have remarkable 

 relations to oxygen, and indeed as we shall see the red corpuscles 

 by virtue of their haemoglobin have a special work in respiration ; 

 they carry oxygen from the lungs to the several tissues. We shall 

 therefore defer the further study of haemoglobin until we have to- 

 deal with respiration. 



The red corpuscle then consists of a disc of colourless stroma 

 with which is associated in a peculiar way the complex coloured 

 body haemoglobin. Though the haemoglobin, as is seen in laky blood, 

 is readily soluble in serum (and it is also soluble in plasma), in the 

 intact normal blood it remains confined to the corpuscle; obviously 

 there is some special connection between the stroma and the haemo- 



flobin ; it is not until the stroma is altered, we may perhaps say 

 illed (as by repeated freezing and thawing), that it loses its hold 

 on the haemoglobin, which thus set free passes into solution in the 

 serum. The disc of stroma when separated from the haemoglobin 

 has as we have just said an obscurely spongy texture ; but we do 

 not know accurately the exact condition of the stroma in the intact 

 corpuscle or how it holds the haemoglobin. There is certainly no 

 definite membrane or envelope to the corpuscle, for by exposing 

 blood to a high temperature, 60 C., the corpuscle will break up 

 into more or less spherical pieces, each still consisting of stroma 

 and haemoglobin. 



The quantity of stroma necessary to hold a quantity of haemo- 

 globin is exceedingly small. Of the total solid matter of a 

 corpuscle more than 90p.c. is haemoglobin. A red corpuscle in 

 fact is a quantity of haemoglobin held together in the form of a 

 disc by a minimal amount of stroma. Hence whatever effect the 

 stroma per se may have upon the plasma, this, in the case of 

 mammals at all events, must be insignificant : the red corpuscle is 

 practically simply a carrier of haemoglobin. 



F. 3 



