512 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



Invertebrated classes in general. These movements are as automatic 

 and involuntary as are the ordinary reflex actions, but differ from them 

 in requiring that the impressions which originate them should be felt as 

 sensations ; and hence they are conveniently designated as consensual. 

 As examples of this group, we may advert to the start upon a loud and 

 unexpected sound ; the sudden closure of the eyes to a dazzling light, 

 or on the approach of bodies that might injure them, which has been 

 observed to take place even in cases in which the eyelids could not be 

 voluntarily closed; the act of sneezing excited by an irritation of the 

 nostril, and sometimes also by a dazzling light; the semi-convulsive 

 movements and the laughter called forth by tickling ; and the vomiting 

 occasioned by the sight or the smell of a loathsome object. So, again, 

 the act of yawning is ordinarily called forth by certain uneasy sensa- 

 tions within ourselves, but also by the sight or hearing of the act as 

 performed by another. Various phenomena of disease exhibit the 

 powerful influence of sensations in producing automatic motions. As 

 instances of this kind, we may refer to the effects of the sight or the 

 sound of liquids, or of the slightest currents of air, in exciting the 

 Hydrophobic paroxysm ; whilst in many Hysteric subjects the sight of 

 a paroxysm in another individual is the most certain means of its induc- 

 tion in themselves. The most remarkable examples, however, of auto- 

 matic movements depending upon sensations, are those which we come 

 to perform habitually, and, as we commonly say, mechanically, when 

 the attention and the voluntary effort are directed in quite a different 

 channel. Thus the man who is walking through the streets in a com- 

 plete reverie, unravelling some knotty subject, or working out a mathe- 

 matical problem, not only performs the movements of progression, which 

 may be simply reflex) with great regularity, but also directs these in a 

 manner which plainly indicates the guidance of sensations. Thus, he 

 will avoid obstacles in the line of his path, and he will follow the course 

 which he has been accustomed to take, although he may have intended 

 to pass along some very different route ; and it is not until his attention 

 is recalled to his situation, that his train of thought suffers the least 

 intermission, or that his will is brought to bear upon his motions. 



905. We may trace the agency of the Sensory Ganglia, however, in 

 the Human subject, not merely in their direct and independent opera- 

 tion upon the muscular system, but also in the manner in which they 

 participate in all Voluntary actions. The existence of a Sensation of 

 some kind, in connexion with a Muscular exertion, seems essential to 

 the continuance of the latter. Our ordinary movements are guided by 

 what is termed the Muscular Sense; that is, by a feeling of the con- 

 dition of the muscle, that comes to us through its own sensory nerves. 

 How necessary this is to the exercise of muscular power may be best 

 judged of from cases in which it has been lost. Thus, a woman who 

 had suffered complete loss of sensation in one arm, but who retained its 

 motor power, found that she could not support her infant upon it, 

 without constantly looking at the child ; and that, if she were to remove 

 her eyes for a moment, the child would fall, in spite of her knowledge 

 that her infant was resting upon her arm, and of her desire to sustain 

 it. Here, the muscular sense being entirely deficient, the sense of 



