THE UNIT OF STRUCTURE 31 



The most remarkable variations in size are to be found 

 amongst the cells of the nervous system. It may be given as 

 one of the most distinctive characters of nervous tissue that 

 its cells have no fixed or standard dimensions. A nerve- cell 

 enters into connection with other nerve-cells and with muscle- 

 fibres by means of branches, or cell-processes, as they are 

 termed. The cells may be globular, as in the sympathetic 

 system, or star-shaped. Each cell gives off a certain number 

 of processes, which divide like the branches of a tree, and one 

 process which may run for a very long distance without 

 dividing. This latter thread-like process places it in com- 

 munication either with a distant part of the central nervous 

 system or with the muscle-fibres which it controls. By means 

 of such a thread a cell in the spinal cord may be connected 

 with muscle-fibres of the hand or of the foot. The thread is 

 really a bundle of filaments (neurofibrillse) which separate to 

 supply a number of muscle-fibres. It is, in its whole length, 

 a part of the cell in which it originates. The size of the cell 

 varies as the number of filaments in this bundle (termed the 

 " axon "), and possibly also as their length. Hence it comes 

 about that nerve-cells may be amongst the smallest, or they 

 may be the very largest, in the body. The so-called " granules " 

 of the cortex of the cerebellum and of the cerebrum are almost 

 as small as red blood-corpuscles (Fig. 23). Each of them has five 

 or six minute branched processes and an exceedingly delicate 

 axon. The large cells of the cerebral cortex, which send their 

 axons far down the spinal cord, and the large cells of the 

 spinal cord which supply the muscles of the body, have a 

 diameter ten or twelve times as great as that of a granule. 

 But larger still are the nerve-cells which supply the electric 

 organs of the torpedo and other electric fishes (p. 295) ; and 

 largest of all are the cells which innervate the curious " fishing- 

 rods " of the strange angler fish (Lophius piscatorius) . It is 

 difficult, owing to their irregular shape, to say how large these 

 cells are ; but they are visible to the naked eye. 



The anatomical' unit of structure is the cell. Cells are the 

 bricks of which the body is built. Some are large, others 

 small, as befits the part which they take in the construction 

 of the body. If the tissue be merely a supporting tissue, con- 

 nective tissue, cartilage, bone, its cells are uniform in size 



