236 LIFE : OUTLINES OF GENERAL BIOLOGY 



one non-striated muscle at least is "voluntary"— the ciliary muscle 

 of the eye, so necessary for focusing on an object. 



Ordinary "voluntary" muscle is made up of cross-striped cells 

 much elongated, and usually with many nuclei, which are some- 

 times situated peripherally, as in man; sometimes embedded in the 

 muscle-substance, as in the frog. A striped muscle-fibre is usually 

 a mucli-clongated cell; but in some cases it seems to be due to the 

 longitudinal fusion of a few cells. 



lleart-muscle is involuntary, but cross striped, thus somewhat 

 intermediate between the other two kinds. Its striai are less marked 

 than those of striped muscle, and each cell has a single nucleus. One 

 cell is connected to another by terminal branches, and their boun- 

 daries are so vague that the whole is often described as a syncytium 

 or coalesced grouping of cells. 



Dkscription of Muscle.— Though every biological student is 

 familiar with muscular tissue, we venture to include here a very 

 simple description of a piece of "flesh". When we look into a piece 

 of flesh we see that it consists of a large number of contractile 

 threads, the muscle-fibres, each complete in itself. In the biceps 

 nuiscle of our arm, by which we bring the lower arm nearer the 

 upper arm, there are about half a million of these muscl^-fibrcs , 

 each an engine condensed into a living thread. 



Each living thread or fibre is an elongated cell, often about 

 an inch long, but so slender, say noo inch in diameter, that it 

 cannot be seen with the naked eye. There is a delicate elastic cell- 

 wall (s\rcolcmma) enclosing a fluid living matter in which are 

 embedded a large number of microscopic filaments or fibrils. These 

 run the whole length of the fibre, and it is their contraction that 

 contracts each fibre and so the whole muscle. Underneath the 

 sarcolcmma, which seems to insulate thread from thread, there are 

 superficial nuclei at irregular intervals. 



An ordinary muscle-fibre highly magnified has a cross-striped 

 appearance, the fibrils showing alternate dark and light portions. 

 Moreover, each light portion is crossed by a darkish line, and each 

 dark portion by a light line, and we know that during contraction 

 the light portion seems to be swallowed up, as it were, by the dark 

 portion, which swells. Here we get a glimj)se into the intricacy of 

 the living engine, on the structure of which histologists are still far 

 from agreed after well nigh a century of microscopic endeavour. 



In the process of contracting there is a shortening and broadening 

 of each fibre and of the muscle as a whole, and as each fibre is con- 

 nected with one of the many threads of a tendon or sinew that is 

 fastened in most cases to a piece of skeleton, the result is drawing 

 this nearer to or farther from another piece. In this way work is 

 done. But this cold statement must be enlivened, if the contraction 

 of muscle is to be appreciated. We must think of a bird like a swift 



