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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



to the needs of the parts to be influenced, and it is found that the structure of 

 the muscles differs according to the work which they have to perform. Thus 

 we find two large classes of muscles : the one, like the muscles which move the 

 bones, remarkable for the rapidity with which they change their form, but 

 unsuited to long-continued action ; the other, occurring in the walls of the 

 intestine, blood-vessels, bladder, etc., sluggish of movement, but possessing great 

 endurance. The first of these, when examined with the microscope, is seen 

 to be composed of bundles of fibres, which are transversely marked by alter- 

 nating dark and light bauds, and hence are called striated or striped muscles ; 

 the other, though composed of fibres, shows no such cross markings, and 

 therefore is known as smooth or non-striated muscle. Striated muscles are 



SL 



Fig. 1.— Amoeba proteus, magnified 200 times: a, endosarc; b, simple pseudopodium ; C, ectosarc; d, 

 first stage in the growth of a pseudopodium : c, pseudopodium a little older than d ; /, branched pseudo- 

 podium; g, food-vacuole ; h, food-ball; i, endoplast; k, contractile vesicle (after Brooks: Handbook of 

 Invertebrate Zoology). 



often called voluntary, because most of them can be excited to action by the 

 will, whereas non-striated muscles are termed involuntary, because in most 

 cases they cannot be so controlled. Within these two large classes of muscles 

 we find special forms presenting other, though lesser, differences in function 

 and structure. The muscle of the heart, though striated, differs so much from 

 other forms of striped muscle as almost to belong in a special class. 



