. The structure and life of birds . t present, it varies fromone fourth to one twenty-fourth. There are two kinds of muscles: (1) striated orstriped ; (2) unstriated or smooth. All muscles whichwe move voluntarily are striated, and it is these whichmove most quickly. The unstriated muscles, on theother hand, which aid in carrying on the processes oflife in the body, move slowly and are subject to thesympathetic system of nerves which are not underthe control of the will. The muscle of the Iris in man isaltogether exceptional ; it is unstriated ; its action is insome cases voluntary, in others
. The structure and life of birds . t present, it varies fromone fourth to one twenty-fourth. There are two kinds of muscles: (1) striated orstriped ; (2) unstriated or smooth. All muscles whichwe move voluntarily are striated, and it is these whichmove most quickly. The unstriated muscles, on theother hand, which aid in carrying on the processes oflife in the body, move slowly and are subject to thesympathetic system of nerves which are not underthe control of the will. The muscle of the Iris in man isaltogether exceptional ; it is unstriated ; its action is insome cases voluntary, in others involuntary, according 142 THE STRUCTURE AND LIFE OF BIRDS chap. as its activity is due to one or another set of nerves ;it moves with great rapidity, instantaneously enlarg-ing or reducing the size of the The muscles ofthe heart, too, are peculiar. Though involuntary, theyare striated, and yet unlike other striated muscles. The amount of contraction possible to a musclevaries with its length ; its strength depends upon its. £///.;I;7/ mm 4jiliiiiiiii B * fiiii iiS Fig. 36 (after Huxley).A, Striated muscle of frog ; B, of mammal, teased out ; c, non-striated muscle. thickness. Thus a short thick muscle will have strength,but no great range ; a long thin muscle, great range,but little strength. It has been found that a musclecannot contract more than one-third of its length. Itwill be important to bear in mind this principle, whenwe come to consider the varying lengths of thebreastbone and, consequently, of the muscles arisingfrom it, in birds with different methods of flight. See p. 127. vi FORM AND FUNCTION 143 Muscle is related to another kind of tissue which yetin its function is very different. Tendons have nopower of contraction. They are. merely cords bywhich, in many cases, the ends of muscles arefastened to the bones. In youth there is compara-tively little tendon in the body, nearly all is muscle,and to this is due the springiness of the limbs. Inage one of tw
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