THE MUSCULAR SYSTEM 399 



This retarded relaxation is termed contracture ; it may be pro- 

 duced in other ways — viz., by repeated electrical stimulation of 

 a muscle. The muscle, while being held in a state of maintained 

 contraction, is undergoing quick contractions and relaxations. 

 It has been supposed that such a condition may not be unfavour- 

 able to a muscle at the beginning of work, though unfavourable 

 when the muscle begins to tire. It is very likely that the con- 

 dition of muscular cramp may be associated with this state of a 

 muscle, and that it represents the bulk of the cases of so-called 

 dislocation of the patella in the horse. 



Muscle Wave. — When the impulse enters a muscle at the 

 middle of each fibre, the part nearest the end-plate contracts 

 first, and the impulse spreads each way to the end of the fibre ; 

 this process is so rapid, the fibre only being, as we have seen, 

 about 1 inch in length, that for all practical purposes the whole 

 muscle contracts at one and the same time. When, however, 

 the nerve-endings in a muscle are paralysed by curare, the part 

 becomes at once practically nerveless, and if under these con- 

 ditions one end of the muscle be stimulated, a wave of contraction 

 passes along it to the other at a rate of about 3 to 4 metres (10 or 

 12 feet) a second in the curarised muscle of the cold-blooded frog. 

 In the muscles of warm-blooded man, where the metabolic 

 processes are more active than in the frog, the rate of propagation 

 is greater, and may be taken as 10 to 13 metres (30 to 40 feet) per 

 second. The time which the wave takes to pass any one point 

 of the muscle is extremely short, T V second ; and if (in frog's 

 muscle) we take its velocity as at least 10 feet per second, a simple 

 calculation shows that the length of the wave is about 1 foot 

 (35 centimetres). This is a fact of great interest in connection 

 with the efficiency of a muscle as a machine. It ensures that 

 each single fibre of which the skeletal muscles are composed, and 

 hence the whole muscle itself, can be placed in a state of complete 

 contraction from end to end at the same moment. The skeletal 

 muscles when exposed immediately after death show irregular 

 contractions, bundles of muscle fibre contracting and relaxing 

 irregularly, producing a flickering, quivering appearance. This 

 is known as fibrillar contraction, and, as we have already seen, it 

 becomes a serious patholpgical condition when it affects the heart 

 muscle. 



Summation. — If instead of passing a single stimulus into a 

 muscle-nerve preparation, two are sent in, so arranged that the 

 second follows the first before the muscle has time to relax from 

 the first contraction, the contraction due to the second stimulus 

 is superposed on the first, and the effect obtained is a stronger 

 total contraction. If a third stimulus be sent in before relaxa- 

 tion occurs from the second, the lever of the muscle preparation 



