THE UNIT OF STRUCTUKE 29 



by many histologists be sound, by cell division, in the ordinary 

 sense of the term, but appear as spots, gradually growing into 

 discs inside the body of a blood-forming cell. The discs are 

 extruded when they reach their full dimensions. Yet the 

 tissue, blood, is composed of these blood-discs and the inter- 

 mediate substance blood-plasm. Mammalian blood might 

 be dismissed as a non-cellular fluid secretion containing formed 

 elements, if it were not for its history. In all animals below 

 mammals the red corpuscles are cells with nuclei and cell- 

 bodies. The absence of nuclei in mammals is due to the 

 recognition by Nature of the fact that, as the blood-cells will 

 never be called upon to divide, it is a waste of material to 

 provide each of them with a nucleus. Not only would the 

 nucleus be useless, but it would take up space, diminishing 

 the capacity of the corpuscle for carrying haemoglobin. The 

 process of cell division is in consequence curtailed. There 

 are, it is true, other ways of looking at this problem. The 

 cells which line the bloodvessels stand in some sort of nutritive 

 relation with the blood. When the lining cells of the blood- 

 vessels are injured or inflamed, the blood clots. But here 

 again it is somewhat straining a point to say that these lining 

 cells are the cells of the blood, and the blood a kind of inter- 

 cellular substance ; especially as a distinction would have to 

 be made between mammals with non-nucleated blood-cor- 

 puscles and birds with complete blood-cells. 



The physiologist, if he is to feel sure of his ground, needs 

 to know the minute anatomy as well as the naked-eye anatomy 

 of the body. But what is there that he does not need to know ? 

 He must be chemist, physicist, biologist, pathologist, and 

 expert in various other branches of science. Microscopic 

 anatomy, or histology, as it is commonly termed, will be 

 called upon in this book only when it has evidence to give 

 which bears directly on physiological problems. We have 

 dwelt at some length upon the cell theory because the 

 physiologist needs starting-points. He needs to have in his 

 mind a conception of the fundamental structure of the body. 

 Protoplasm is the material which lives. We begin with 

 protoplasm albeit our conception of protoplasm is so difficult 

 to formulate that we are obliged to admit that in using the 

 term we are almost guilty of playing with words. Protoplasm 



