28 THE BODY AT WORK 



united by nerve filaments that the nerve- cell and muscle- cell 

 grow apart without severing this thread-like connection. 

 Certain anatomists regard the nerve strand which connects a 

 cell in the central nervous system with a number of muscle- 

 fibres, placed, it may be, at a great distance from the nerve- 

 cell, as the bridge which has never been broken in the process 

 of cell division and displacement, which made one primitive 

 cell into a nerve-cell and a group of muscle- cells. Muscle-fibres 

 are not separate cells, but cell complexes. Each muscle- fibre 

 contains scores, in some cases hundreds, of nuclei (Fig. 16). It 

 is a cylinder, perhaps 2 inches long, in which cell division is 

 incomplete. Tendons are bundles of exceedingly slender fibres 

 which lie side by side, like silk threads in a skein. The row of 

 cells which gives rise to a tendon undergoes incomplete cell 

 division. Their nuclei divide, and a small quantity of soft 

 body- substance is set apart for each nucleus. The rest of the 

 mass consists of fused cells. It constitutes a continuous rod, 

 which becomes fibrillated as it grows. Vegetable cells are 

 separated by cell- walls. Animal cells tend to develop inter- 

 mediate partitions ; but the partitions are so thick that they 

 can no longer be described as walls. In cartilage the cell- 

 bodies are embedded in a great mass of intercellular substance, 

 or matrix. In this intercellular substance elaborate develop- 

 ments may take place. Elastic fibres may make their appear- 

 ance in it to form elastic cartilage, as in the case of the 

 epiglottis. In these various instances, although it is perfectly 

 true that tissues are formed by cell division, the cells are not, 

 strictly speaking, separate units. They are not completely 

 divided one from another. It is impossible to recognize their 

 anatomical boundaries. 



But there is a much more serious difficulty in applying the 

 cell theory the difficulty of deciding what are the essential 

 parts of a cell. Long ago it was recognized that many animal 

 cells white blood-corpuscles, for example have no cell-wall. 

 It was therefore decided that cell-body and nucleus are the 

 only essential parts. But what is to be said of the red blood- 

 corpuscles of mammals ? (Fig. 4). Are they cells ? They have 

 neither cell- walls nor nucleus ; nor does their substance present 

 the structure which is usually associated with the " body- 

 substance " of cells. They are not produced, if the view held 



