910 OP MUSCULAR MOVEMENTS. 



life, we find that they lose more and more of the character they possess in Man, 

 becoming more and more exclusively automatic, and at last being even trans- 

 ferred from the more elaborate mechanism of muscular contraction, to the simple 

 operation of ciliary vibration. 1 Nearly all those muscles in the Human body, 

 which are ordinarily called into action by the Cranio-Spinal nerves, are composed 

 of striated fibre ; the most remarkable exception being the muscular structure of 

 the Iris. And it is peculiarly characteristic of them that, whilst forcible and 

 united contractions of all the fasciculi at once are called forth by irritating 

 their nerves, the effect of direct stimulation is limited to the fasciculus irri- 

 tated. 



913. It is obvious, from what has preceded, that the system of classifying the 

 Muscles under the categories of voluntary and involuntary, cannot be consist- 

 ently maintained. It is quite true that all the Muscles of Organic Life may 

 be truly styled " involuntary " for, although they are capable of being influenced 

 by emotional and ideational states of mind ( 923), yet the Will cannot exert 

 any direct influence upon them, only affecting them indirectly by its power of 

 determining these states. But over those Muscles, also ministering to the Or- 

 ganic functions, and doing so in obedience to impulses purely automatic, which 

 are called into action by the Cranio-Spinal nerves, the Will, as we have seen, 

 exerts some power ; and such, therefore, cannot be properly regarded as involun- 

 tary, since the Will can influence their actions ; whilst they are far from being 

 truly voluntary, since the Will cannot control their tendency to automatic action 

 beyond a certain limited amount. On the other hand, every one of the Muscles 

 usually styled voluntary, because ordinarily called into action by the Will, is 

 liable to be thrown into action involuntarily ; either by an Excito-motor stimulus, 

 as in tetanic convulsions, or by Consensual action, as in tickling, or Emotion- 

 ally, as in laughter or rage, or simply Ideationally, as in somnambulism and 

 analogous states. Hence, although there are certain groups of muscles which 

 are more frequently acted on by the Will than by any other impulse, and 

 certain others which are more frequently played on by the Emotions, and so on, 

 it becomes obvious that every muscle called into contraction by the Cranio-Spinal 

 nervous system is capable of receiving its stimulus to movement from any of 

 these sources j the nerve-force transmitted along the motor fibres being issued 

 either from the Spinal Cord, from the Sensory Granglia, or from the Cerebrum, 

 as the case may be, but being in its nature and effects the same in every in- 

 stance. 



914. The grouping or combination of Muscular actions, which takes place 

 in almost every movement of one part of the body upon another, must be at- 

 tributed, not to any peculiar sympathy among the Muscles themselves, but to 

 the mode in which they are acted on by the Nervous Centres. This is most ob- 

 viously the case with regard to those of the primarily automatic class ; but it 

 cannot be in the least degree doubtful, as to those of the secondarily automatic 

 kind (such as walking), which, though at first directed by the Will, come by 

 habit to be performed under conditions essentially the same with the preceding; 

 and when it is borne in mind that even in voluntary movements the Will cannot 

 single out any one muscle from the group with which it usually co-operates, so 

 as to throw this into separate contraction, but is limited to determining the 

 result ( 758), it seems pretty obvious that even here the grouping is efl'ected 

 by the endowments of the Automatic centres from which all the motor impulses 

 immediately proceed to the muscles, and not by Cerebral agency. In fact, 

 the whole process by which we acquire the power of adapting our muscular 



1 Thus in the Oyster and other Bivalve Mollusks, which have a complicated digestive, 

 circulating, and respiratory apparatus, food is brought to the mouth, fecal matters are ex- 

 pelled from the anus, and a constant current of water is made to sweep over the respiratory 

 surface, entirely by ciliary motion. 



