228 NERVE CELLS AND FIBRES. 



act, and I have shown that between the so-called con- 

 tractility manifested by various forms of living bioplasm 

 and muscular contractility there is an essential difference.* 



In young muscular tissue the proportion of bioplasm is 

 considerable, and at an early period of development greatly 

 exceeds in amount the contractile tissue. The particles of 

 bioplasm (generally known as " nuclei " of the muscle) are 

 very close together, but as the muscle grows they become 

 removed farther and farther from one another until, in 

 many specimens of fully formed muscular tissue, they are 

 separated by very wide intervals. In some fully formed 

 muscular fibres the masses may be separated from one 

 another by as much as the y^-th of an inch, and the observer 

 might be led, from their sparing number in adult muscle, 

 to conclude that they were altogether absent, or that their 

 presence here and there was accidental and unimportant ; 

 but if the muscular tissue were examined in a young animal 

 of the same species, they would be found in great number. 

 In the muscles of some animals, which undergo great 

 changes in nutrition at different periods of the year, the 

 bioplasts vary in number in contiguous elementary fibres. 

 In the batrachia, in spring time, even in adult animals 

 muscular fibres may be found in every stage of development, 

 and the development of muscle may be studied as success- 

 fully as in the embryo. 



The Formation of Central and Peripheral Nerve Cells 

 and Nerve Fibres. Although the large central nerve cells 

 (spherical, oval, caudate) of the adult are so very different 

 in size, structure, and appearance, from the peripheral 



* "On Contractility as distinguished from purely vital movements." 

 "Trans. Mic. Soc." 1866. 



