122 ZOOLOGY. 



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

 the brain, and Mowing thence into the muscles, was th<- etli- 

 cicnt cause of muscular contraction ; a hypothesis incompatible 

 with certain facts recent ly oh>erved. the nervous system is, 

 no doubt, the determining cause of muscular contraction. 



253. Muscles present very important differences amongst 

 themselves: sunn- 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, ditfer 

 generally from the involuntary, in being marked with trans- 

 verse strise; 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 transver 

 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 

 all the muscles. Injury done to the corpus striatum renders 

 the muscular movements confused and uncertain ; the power 

 of retiring is lost, the movements being always in advance : 

 an injury done to the cerebellum (in birds) seems, on the 

 contrary, to disable the animal from advancing ; its move- 

 ments are constantly backwards. 



By dividing vertically in a living animal one hemisphere 

 of the cerebellum, or one side of the pons, the animal moves 

 incessantly round and round towards the injured side. The 

 vital point of the encephalon seems to be close to the origin 

 of the pneuinogastric nerves. The cerebellum and the ad- 

 joining parts of the brain seem to regulate the movements of 

 locomotion. 



The movements which, though obeying the will, yet take 

 place independently of its influence, seem to be derived, then, 

 from the influence of the spinal marrow. The respiratory 

 movements, for example, go on after the brain has lost its in- 

 fluence over the respiratory muscles. 



256. Influence of fin (jimytionarii tys/an. The mus- 

 cular fibres, which are completely independent of the will. 

 receive their nervous influence from this system. 



