REFLEX ACTIONS. MOVEMENTS OF LOCOMOTION. 307 



of irritation ( 381). It Avill hereafter be shown that the stimulus is conveyed, 

 in this case, not through the Olfactory nerve, but through the Fifth pair; so 

 that it is not dependent upon the excitement of the sensation of Smell. The 

 act of Coughing, also, may be regarded as of a protective character ; being 

 destined to remove sources of irritation from the air-passages. The automatic 

 movements, performed by the limbs of Frogs and other animals, when their 

 connection with the brain has been cut oft" ( 306, 370) appear destined to 

 remove these parts from sources of irritation or injury; and they may thus be 

 rightly placed under the same category. 



397. Movements of Locomotion. Lastly, we have to inquire how far the 

 Reflex function of the Spinal Cord is concerned in the locomotive actions of 

 the lower extremities in Man. It will be remembered that, in the Dytiscus 

 whose head had been removed ( 328), the stimulus of the contact of water 

 immediately excited regular and continued locomotive actions which lasted 

 for some time. So in the cases already quoted ( 366 368), when the con- 

 trol of the will over the lower extremities was lost, powerful muscular actions 

 were excited in them, through the Spinal Cord alone. In the healthy con- 

 dition of the Human system, when the Will is controlling all the movements, 

 which are not immediately concerned in the maintenance and regulation of 

 the organic functions, no such actions can be excited : but in proportion as its 

 control is lost, does the independent power of the Spinal Cord manifest itself. 

 The more such actions are of a simple rhythmical character, similar to those 

 of Respiration, the more does it seem that they may with probability be re- 

 ferred to the Spinal system; and if we attribute to this (as we can scarcely 

 help doing) the rapid vibration of the wings of Insects, there seems no reason 

 why we should not extend the same view to the wings of Birds. Such an 

 explanation of their movements will account for their occasional continuance, 

 without apparent fatigue, during a period through which no known voluntary 

 effort can endure ; for it is one of the attributes of the Spinal system of 

 nerves, well pointed out by Dr. M. Hall, that the exercise of the muscles 

 excited by it does not occasion fatigue, the sense of which is Cerebral only. 

 It would seem to the A.uthor more probable, however, that those movements 

 which guide the body, and which must themselves be directed by Sensation, 

 are to be referred to a class intermediate between the Voluntary and the Re- 

 flex, which may be properly termed Consensual. Numerous actions, in Man, 

 which were at first Voluntary, appear at last to be performed as instinctively 

 or intuitively, as they are in the lower animals from the commencement of 

 their existence. (See the next Section.) 



398. Influence on Muscular Tension. The various muscles of the body, 

 even when there is the most complete absence of effort, maintain, in the 

 healthy state of the system, a certain degree of firmness, by their antagonism 

 with each other ; and if any set of muscles be completely paralyzed, the op- 

 posing muscles will draw the part on which they act, out of its position of 

 repose ; as is well seen in the distortion of the face, which is characteristic 

 of paralysis of the facial nerve on one side. This condition has been desig- 

 nated as the tone of the Muscles ; but this term renders it liable to be con- 

 founded with their tonic contraction, which is also concerned in maintaining 

 their firmness, but which operates in a very different manner. The latter is 

 dependent upon the simple contractility of the muscle ; and is exhibited alike 

 by the striated and the non-striated forms of muscular fibre, but more espe- 

 cially by the latter ( 593). On the other hand, the condition now alluded to, 

 which may perhaps be appropriately termed their tension, is the result of a 

 moderate though continued excitement of that contractility, through the nerv- 

 ous centres. It has been proved by Dr. M. Hall, that the Muscular Tension 

 is not dependent upon the influence of the Brain ; but upon that of the Spinal 



