MUSCLES. 57 



35. Although the normal stimulus to muscular contraction 

 is derived through nerves, the muscles may be excited to 

 contract by various other stimuli, mechanical, chemical, and 

 electrical, and by heat and cold. Isolated muscular fibres, 

 both striped and unstriped, have been made to contract 

 under the microscope. The irritability of muscular fibre is, 

 therefore, inherent in itself, and not due to its nervous con- 

 nections. Complete contraction sustained for a short time 

 is followed by a condition of exhaustion or temporary loss of 

 irritability; but the duration of contractile power may be 

 greatly increased in the pathological spasm termed tetanus, 

 the condition which constitutes lockjaw when it affects the 

 muscles of mastication. However, a certain slight amount 

 of habitual contraction of a continuous description, distin- 

 guished as tonicity, exists in a number of muscles, possibly 

 in all ; and we shall find it illustrated in the coats of arteries, 

 and in the circular muscles called sphincters, which keep the 

 orifices around which they are situated closed ; for example, 

 the pyloric orifice of the stomach. 



The irritability of muscle continues for some time after 

 the death of the animal, and in some cold-blooded animals 

 may continue for days. The properties of living muscle can 

 therefore be studied on parts separated from the bodies of 

 animals. A block of living muscle placed in the circuit of a 

 galvanometer exhibits a remarkable description of electric 

 tension so long as it is quiescent. A galvanometer is an 

 instrument which indicates the presence and direction of 

 electric currents in a wire by means of the deviations of 

 a magnetic needle placed over an insulated coil. If the 

 circuit of the wire be completed by making contact with 

 the transversely cut extremities of the block of muscle, 

 or with points equally distant from the centre, no cur- 

 rent is exhibited; but if the contact be made with any 

 other points, the galvanometer indicates the passage of a 

 current through the wire from the point nearest the centre 

 of the block of muscle to the other; and this current is 

 strongest when one point of contact is at the centre of the 

 block, and the other at one extremity. If, however, the 

 muscle be made to contract, the condition of electric tension 

 ceases. 



