THE CONTRACTILE TISSUES. 176 



light, and be invested witli a breadth of meaning they would 

 not otherwise possess. 



The Irritability of Muscle and Nerve,— An animal, as a frog, 

 deprived of its brain, will remain motionless till its tissues have 

 died, unless the animal be in some way stimulated. If a mus- 

 cle be isolated from the body with the nerve to which it be- 

 longs, it will also remain passive ; but, if an electric current be 

 passed into it, if it be pricked, pinched, touched with a hot body 

 or with certaia chemical reagents, contraction ensues ; the same 

 happening if the nerve be thus treated instead of the muscle. 

 The changes in the muscle and the nerve will be seen later to 

 have much in common ; the muscle alone, however, contracts, 

 undergoes a visible change of form. 



>^ 



Fig. 1B9.— Intraflbrillar terminations of the motor nerve in striated muscle, stained 

 with goJd chloride (Landois). 



Now, the agent causing this is a stimulus, and as we have 

 seen, may be mechanical, chemical, thermal, electrical, or nerv- 

 ous. Ab both nerve and muscle are capable of being function- 

 ally affected by a stimulus, they are said to be irritable ; and 

 since muscle does not contract without a stimulus, it is said to 

 be non-automatic. 



Now, since muscle is supplied with nerves, as well as blood- 

 vessels, which end in a peculiar way (end plates) beneath the 

 muscle-covering (sarcolemma) in the very substance of the pro- 

 toplasm, it might be that when muscle seemed to be stimu- 

 lated, as above indicated, the responsive contraction was really 

 due to the excited nerve terminals ; and thus has arisen the 

 question. Is muscle of itself really irritable ? 



What has been said as to the origin of muscular tissue points 

 very strongly to an aflSrmative answer, though it does not fol- 

 low that a property once possessed in the lower forms of a tissue 

 may not be lost in the higher. From various facts it may be 

 concluded that muscle possesses independent irritability. 



