On muscular Motion. 223 



tions, are known to exhaust the muscular strength : intense 

 thought and anxiety. weaken the muscular powers, and the 

 passions of grier' and iVar produce the same effect suddenly j 

 whilst the contrary feelings, such as the prospect of imme- 

 diate enjoyment, or moderate hilarity, give more than or- 

 dinary vigour. 



It is a very remarkable fact in the history of animal na- 

 ture, that the mental operation's may become almost auto- 

 matic, and, under such habit, be kept in action, without 

 any interval of rest, far beyond the time which the ordinary 

 state of health permits, as in the examples of certain ma- 

 niacs, Vi'hoare enabled, without any inconvenience, to exert 

 both mind and body for many days incessantly. The ha- 

 bits of particular modes of labour and exercise are soon ac- 

 quired, after which the actions become automatic, demand 

 little attention, cease to be irksome, and aro affected with 

 little fatigue : by this happy provision of nature, the habit 

 of industry becomes a source of pleasure, and the same ap- 

 pears to be extended to the docile animals which co-operate 

 with man in his labours. 



Three classes of.muscles are found in the more compli- 

 cated animals. Those which are constantly governed by 

 the will, or directing power of the mind, are called volun- 

 tary muscles. Another class, which operate without the 

 consciousness of the mind, are denominated involuntary: 

 and a mixed kind occur in the example of respiratory mus- 

 cles, which are governed by the will to a limited extent : 

 nevertheless the exigencies of the animal feelings eventually 

 urge the respiratory movements in despite of the will. 

 These last muscles appear to have become automatic by the 

 continuance of habit. 



The uses of voluntary muscles are attained by experience, 

 imitation, and instruction ; but some of them are never 

 called into action among Europeans, as the muscles of the 

 external ears, and generally the oecipilo-frontalis. The 

 purely involuntary muscles are each acted upon by different 

 substances, which appear to be their peculiar stimuli; and 

 these stimuli co-operate with the sensorial influence in pro- 

 ducing their conUaetions : for exaniple, the bile appears to 

 be the appropriate stimulus of the nuiseular fibres of the 

 alimentary canal below the stomach, because the absence 

 of it renders those passages torpid. The digested aliment, 

 or perhaps the gastric jui';c in a certain state, excites the 

 stomach. The blood stimulates the heart, light the iris of 

 the eye, and mechanic;d pressure seemr, to exeile ,the mus- 

 cles of the cesophagU'^. 'J'he last cause may pcrh:>ps be il- 



Vol. 23. No. yi. Dvc. ISUJ. P luilrutgJ 



