128 ZOOLOGY. 



[The true aponeuroses envelope the muscles of the limbs, and 

 form, as it were, a system apart from the tendons. R. K.] 



250. Influence of the Nervous System on Muscular 

 Contraction. The muscles are the only parts of animals 

 which possess the faculty of contracting ; but this property is 

 displayed only through the influence of the nervous system. 



251. Influence of the Nerves. Each muscular fasci- 

 culus receives several nervous filaments. These proceed 

 parallel with the fibres enclosed in a neurilemma : they also 

 pass transversely across the muscular fibres and form loops, 

 but how they terminate is not known. They seem to form a 

 continuous circle. 



By cutting the nerve across which supplies a muscle in a 

 living animal, the muscle becomes paralysed. By compressing 

 the brain of a living animal, all power of motion is lost. 



252. The nature of this influence of the nerves over the 

 muscles has been much investigated ; and the inquiry led 

 Galvani and Volta to some brilliant discoveries. They proved 

 that electric currents, however produced, act on muscles in a 

 similar way to the will, producing contractions in them even 

 after death ; and so closely did the phenomena resemble each 

 other, that it was at first thought that electricity, produced in 

 the brain, arid flowing thence into the muscles, was the effi- 

 cient cause of muscular contraction a hypothesis incompatible 

 with certain facts recently observed. The nervous system is, 

 no doubt, the determining cause of muscular contraction. 



253. Muscles present very important differences amongst 

 themselves : some obey the will ; others, in addition to this, 

 also act independently of it ; whilst there are others over 

 whose acts the will has no power. The muscles of the limbs 

 may be cited as instances of the first class ; those of respiration 

 belong to the second ; the heart and stomach to the third. 



The muscles of voluntary motion, as they are called, differ 

 generally from the involuntary, in being marked with trans- 

 verse striae ; but the fibres of the heart have these striae, 

 although it is in no respect a voluntary muscle. 



254. The muscles obeying the will all derive their 

 nerves from the cerebro-spinal axis. 



255. Influence of the Encephalon. A transverse sec- 

 tion of the spinal marrow in a living animal paralyses the 

 action of all the muscles supplied by nerves which leave the 

 spinal marrow below the section ; those above the section 

 remain uninfluenced. The destruction of the brain paralyses 



