MUSCLES AND PHENOMENA OF CONTRACTION. 119 



on account of their shape, spindle cells. These cells when 

 occasions seem to demand elongate, their nuclei multiply, 

 the external layer develops the sarcolemma and the interior 

 protoplasm, at first structureless, receives gradually the 

 characteristic cross markings. These cross marks seem to 

 appear first toward the outside, and later on extend through 

 the middle. The nuclei at first distributed through the pro- 

 toplasm take their position under the sarcolemma, and in 

 this way a new muscle fibre has been produced. A sec- 

 ond method of muscle formation is claimed, in which the 

 new fibres arise by a longitudinal splitting of old ones. 

 There is no doubt but what under the microscope such 

 splitting fibres may be occasionally seen, but whether these 

 are fibres in process of formation, or whether they may not 

 be fibres that are undergoing degeneration, is a question 

 still unsolved. 



THE FINER STRUCTURE OF THE MUSCLE FIBRE. 



The histological points mentioned so far may be made 

 out without any question by any good observer possessed 

 of a good compound microscope. But when we have seen 

 the muscle composed of alternating light and dark bands, 

 it has not yet been made out to what kind of a structure 

 this appearance is due. On account of the extreme mi- 

 nuteness it is difficult to establish the ultimate structure very 

 plainly, so that what is given in explanation of its finer 

 anatomy is to a large extent theory. 



A single fibre is easily seen to consist of still finer fib- 

 rils, as is evidenced by the fact that the ends of a fibre will 

 fibrillate out into delicate fibrils, much like an ordinary 

 twisted rope when its end is unwound. These finer threads 

 are called fibrils, and bundles of them are believed to make 

 each individual muscle fibre. But when subjected to cer- 

 tain chemical agents a muscle fibre may also be made to 

 split horizontally into small disks somewhat like the stack 

 of coin referred to, might be thrown over and fall into the 

 individual pieces. This would seem to show that the fibrils 



