SENSIBILITY OF MUSCLES. 295 



person learning to play on a musical instrument, finds a difficulty in causing 

 the two shorter fingers to move independently of each other and of the rest ; 

 this is particularly the case in regard to the ring-finger. Any one may satisfy 

 himself of the difficulty, by laying the palm of the hand fiat on a table, and 

 raising one finger after the other, when it will be found, that the ring-finger 

 cannot be lifted without disturbing the rest evidently from the difficulty of 

 detaching the action of that portion of the extensor communis digitorum, by 

 which the movement is produced, from that of the remainder of the muscle. 

 Yet to the practised musician, the command of the will over all the fingers 

 becomes nearly alike ; and it can scarcely be doubted, that some change takes 

 place in the structure of the muscle, which favours the isolated operation of 

 its several divisions. 



f VII. Sensibility of Muscles. 



399. Muscles are much less sensible to external impressions than many 

 other parts of the body ; this is seen in amputations, in which the severest 

 pain caused by the section is that of the incision through the skin. It is well 

 known that a needle passed through the skin may be carried into the sub- 

 stance of a muscle with scarcely any further pain ; and the heart laid bare 

 has been observed to possess but a very slight degree of sensibility. This is 

 easily accounted for by our knowledge of the distribution of the nerves ; for 

 although every principal trunk may contain motor and sensory nerves in 

 equal proportion, we know that in its various subdivisions these may be very 

 unequally distributed. Thus, in the third division of the Fifth pair, it has 

 been ascertained that the fibres from the motor root chiefly pass into the mus- 

 cular branches, whilst those of the sensory root predominate in those supplying 

 the surface ; and in the Par Vagum, a difference in the endowments of the 

 several branches has been equally substantiated. Again, in the Orbit we find 

 some muscles supplied by nerves which are exclusively motor, and scarcely 

 receiving any sensory branches from the Fifth pair. Knowing as we do, 

 that the general surface of the body would not derive any advantage from 

 receiving the motor division of the Spinal nerves, and must, on the contrary, 

 be largely supplied from the sensory, it cannot be doubted that, with regard 

 to the subjacent muscles, the case is entirely the reverse. 



400. Muscles are endowed, however, with the power of originating sensa- 

 tions indicative of their ow^n condition; and these sensations differ so far from 

 those conveyed by the usual sensory organs, that it has been proposed to 

 designate the channel through which they are received by a peculiar name, 

 the Muscular Sense. It may be questioned, however, whether this is de- 

 sirable. The property by which we estimate the force with which muscles are 

 contracting, which enables us to compare different degrees of weight and 

 resistance, and to acquire a knowledge of the distances and relative positions 

 of bodies brought in contact with the surface, appears to be the same with 

 that by which we become conscious of fatigue from continued muscular 

 action, and experience pain \vhen the muscles are spasmodically contracted, as 

 in ordinary cramp. Of the importance of this sense in guiding and regulating 

 muscular contraction, instances have already been given ( 257). It is, in 

 addition, one of the principal means by which we acquire our notions of the 

 external world. All our ideas offeree and of resistance are derived from it. 

 When we put our muscles in action to raise a weight, or to push from us an 

 obstacle, we should have no idea without this sense of the effort required; and 

 it is obvious on a little consideration that no accounts of any natural forces 

 (such as that of Gravitation, Magnetic Attraction, &c.) could convey a distinct 

 idea to our minds, were it not for our means of estimating them by the same 



