CHAP. vii. J STIMULI TO MUSCULAK CONTRACTION. 173 



tract by the immediate influence of the nervous tubules distri- 

 buted among them ; and this influence, however called into play, 

 should be styled the nervous stimulus, or the vis nervosa. This 

 nervous stimulus, then, is simply the effect of such a condition 

 of the motor nerves as enables them to induce contraction in mus- 

 cular fibres which are in the due relation to their terminal loops. 

 Of the nature of this condition, and of the mode of its production, 

 we are as completely ignorant, as we are of the nature of all those 

 other conditions of the nervous system on which the manifestation 

 of its various phenomena depends ; but we know some little of the 

 agents by which the nerves are thrown into this state. The chief 

 of them are volition, emotion, and impressions carried by the affer- 

 ent nerves to the nervous centres, and there affecting the efferent, 

 or motor nerves, independently of volition or consciousness ; but to 

 these are to be added various impressions from diseases and injuries 

 of the motor nerves, either at their origin or in their course, together 

 with pressure, heat, chemical substances, electricity, &c., applied to 

 their texture. The former are the natural excitants of the nervous 

 stimulus in the living body ; the latter may be proved to possess 

 this property by observation, and by experiments on nerves distri- 

 buted to muscles, either in the body, or soon after their removal 

 from it. The power of inducing contraction in the muscles is an 

 endowment of those nerves only which have a certain organic con- 

 nexion with the muscles; and these nerves are, therefore, distin- 

 guished as motor. 



There are other stimuli of muscle besides the vis nervosa, which 

 occasion contraction in the living body ; but, in general, these affect 

 only the hollow muscles. Experiment has, indeed, shewn these 

 muscles to be under the influence of motor nerves derived from the 

 spinal marrow ; but it seems probable that some of them, at least, 

 are normally excited to contract by the stimulus of stretching or 

 distension, to which they are peculiarly liable from their arrange- 

 ment as investments to hollow viscera. Muscles have not the 

 capacity of elongating themselves that has sometimes been ascribed 

 to them : when once contracted, they remain shortened, notwith- 

 standing the contractile force have subsided, unless their ends be 

 drawn apart by some extraneous force. This force is that which 

 has been already specified as being always exerted in opposition to 

 active contraction in the living body. In the case of the voluntary 

 muscles, this force always continues to act after the active contrac- 

 tion has ceased ; but in the case of the hollow muscles, where it 



