428 GENERAL ANATOMY. 



intermediate class of mixed movements. It is in fact extreme- 

 ly difficult to establish a well marked distinction between the 

 voluntary movements, tWlt is to say, those under the perfect 

 control of the will, and the involuntary movements; for on the 

 one hand, there are few functions over which the will, and 

 above all the passions, do not exercise their empire; and on 

 the other, many of the voluntary movements become, by as- 

 suetude, almost involuntary; such, for example, are the move- 

 ments of the members which take place without conscious- 

 ness, and without exercise of the will during sleep; such are 

 those of the eyelids which take place even in opposition to the 

 will, when a foreign body approaches the eye; the difficulty 

 or the impossibility of moving simultaneously the superior or 

 inferior members, or the eyes, in a direction opposed to that 

 which they are ordinarily accustomed to, are moreover of this 

 character. The accidental irritation of the muscles, of the 

 nerves, or of the nervous centre, sometimes renders the con- 

 traction of the exterior muscles altogether involuntar} r ; other 

 affections render them immoveable in spite of the will. As to 

 the influence of the will on the movements regarded as in- 

 voluntary, it is evident in those of respiration, of vomiting, 

 and .ruminating; it even appears to be sometimes extended to 

 the movements of the heart, to those of the uterus, the iris and 

 the skin; it is true, that the influence of the passions on the 

 will itself must not be forgotten. 



The movements that have been regarded as mixed, are more 

 especially those which, occurring in. general unconsciously, 

 and without the exercise of the will, may be modified by the 

 latter; of this character are those of the diaphragm. This name 

 is not so generally given to those which being habitually 

 voluntary, are exercised by habit and association, without be- 

 ing directed by the will, as movements by which the superior 

 members are balanced in. walking. 



It is to be remarked that apoplexy and the other cerebral 

 affections generally paralyze the voluntary muscles only. 



686. The varied muscular movements which take place in 

 the living body, are. in general either connected with each 

 other, in order to produce an identical action, or opposed to 



