32 THE BODY AT WORK 



and small. If it have functions to perform which in some 

 cases are carried out best by small cells, in other cases by 

 large ones, the cells are adapted in size to the work that they 

 have to do. Of the various kinds of wandering cells, some 

 the bone-forming cells (osteoblasts), for example are small ; 

 others the bone-eating cells (osteoclasts) relatively large. 

 Nerve-cells, like telephone exchanges, are large or small accord- 

 ing to the size of the area which each supplies. 



All animals of complex organization, from starfishes and sea- 

 urchins to Man, are inhabited by motile cells. In addition to 

 the bricks which enter into the construction of its fabric, each 

 fixed in its place and definitely united to its neighbours, the 

 animal contains leucocytes which wander through its tissue- 

 spaces or float down the streams of lymph or blood. We are 

 disposed to speak of these wanderers as inhabitants of the body, 

 to distinguish them from the elements which enter into the con- 

 struction of their habitation. It is difficult to avoid the 

 temptation of describing the body as a habitation. Allegorical 

 as Aristotle's distinction between body and soul between the 

 habitation and that which inhabits may seem, when contrasted 

 with the exact language of modern science, it would save many 

 a periphrasis if we might still use the monosyllable " soul." 

 The fixed tissues constitute a unity, bound together by nerves. 

 The work done by glands and muscles is done in response to 

 directions conveyed by nerves. It is impossible to say where 

 the control of the nerves ceases to point out any fixed tissue 

 which is not co-ordinated with other tissues, nor susceptible 

 to the influence of the environment as impressed upon the 

 central nervous system, through the medium of sense-organs. 

 The fixed tissues constitute a habitation for the " soul." They 

 share in a common life. The wandering cells are as indepen- 

 dent of control as the parasites which occasionally find entrance 

 into the body. Each must have a soul of its own in Aristotle's 

 sense. Like parasites, they carry on all the business of nutri- 

 tion, respiration, cell division, without reference to the needs 

 of the fixed tissues. They take what they require from the 

 lymph as it leaves the intestines loaded with the products of 

 digestion ; they take it from the lymph in the tissue-spaces ; 

 they take it from the blood. When nutriment or oxygen runs 

 short, they do not share the privations of the fixed tissues. 



