346 THEORETICAL BIOLOGY 



the familiar twofold structure, — a mechanical portion, which 

 shares in the performance of work by the organs, and a proto- 

 plasmic portion, which carries out the repairs. This residuum 

 of protoplasm is the means employed by the control to make 

 injured organs once more capable of functioning. 



From this it appeared that the cellular structure of the 

 organs and tissues was sufficiently established, and the fact 

 that remote portions of the body, such as the lens of the eye, 

 while consisting of cells, yet behaves like a polished, homo- 

 geneous lens of glass, seemed to raise no further problems. 

 The skin, consisting of thousands of cells, and with it the 

 connective tissue, divided up according to the mechanical 

 requirements of the body-machine into strands or flat surfaces, 

 works like a lens, quite irrespective of the number of cells 

 that go to form the parts. 



But in other respects the cellular structure of the tissues 

 is exploited to its utmost by the body-machine. The single 

 muscle-fibres are connected by single nerve-fibres to their 

 representatives, which are able to stimulate the muscle-fibres 

 one at a time ; just as the keys of a piano cause the strings 

 to sound singly. And each single muscle-fibre can influence 

 its representative, which, as each fresh excitation comes in, 

 can lock or unlock, according to the state of tension of the 

 muscle-fibre. Even in this case we might continue to regard 

 muscle-cell, nerve-fibre and the motor-centre serving as repre- 

 sentative as an isolated part of a machine. Indeed, the 

 comparison has even been pushed so far that the whole reflex- 

 arc, from the receptive sense-cell to the effector muscle-cell 

 or gland-cell, has been described as a self-contained electrical 

 apparatus. 



But in this respect the comparison of the community-being 

 with an organism requires caution. As we see, the transfer- 

 ence of excitation from one individual to another often takes 

 place quite automatically, as in a machine, without the 



