40 BLOOD. 



under high powers an obscurely spongy or reticular structure. These color- 

 less, thin discs seen flatwise often appear as mere rings. The substance 

 composing them stains with various reagents and may thus be made more 

 evident. 



The red corpuscle, then, consists obviously of a colorless framework, with 

 which in normal conditions a red coloring matter is associated ; but by 

 various means the coloring matter may be driven from the framework and 

 dissolved in the serum. 



The framework is spoken of as stroma; it is a modified or differentiated 

 protoplasm, and upon chemical analysis yields proteid substances, some of 

 them at least belonging to the globulin group, and other matters, among 

 which is a peculiar complex fat called lecithin, of which we shall have to 

 speak in treating of nervous tissue. 



The red coloring matter which in normal conditions is associated with 

 this stroma is called haemoglobin, and may by proper methods be split up 

 into a proteid belonging to the globulin group, and into a colored 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 colorless stroma with which 

 is associated in a peculiar way the complex colored 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 con- 

 fined to the corpuscle; obviously there is some special connection between 

 the stroma and the haemoglobin ; it is not until the stroma is altered, we may 

 perhaps say killed (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 haemo- 

 globin. 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 haemoglobin is 

 exceedingly small. Of the total solid matter of a corpuscle more than 90 

 per cent, is haemoglobin. A red corpuscle in fact is a quantity of haemo- 

 globin 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. 



25. The average number of red corpuscles in human blood may be 

 probably put down at about 5 millions in a cubic millimetre (the range in 

 different mammals is said to be from 3 to 18 millions), but the relation of 

 corpuscle to plasma varies a great deal even in health, and very much in 

 disease. Obviously the relation may be affected (1) by an increase or de- 

 crease of the plasma, (2) by an actual decrease or increase of red corpus- 

 cles. Now, the former must frequently take place. The blood, as we have 

 already urged, is always being acted upon by changes in the tissues and, 

 indeed, is an index of those changes ; hence the plasma must be continu- 

 ally changing, though always striving to return to the normal condition. 

 Thus when a large quantity of water is discharged by the kidney, the 



