412 MUSCULAR MOTION. 



writers. It appears to be an internal sensation, produced by the 

 muscle pressing on the sensible parts surrounding it, which convey the 

 sensation to the brain. It is by this muscular sense that the brain 

 learns to adapt the effort to the effect to be produced. Without it no 

 precision could exist in the movements of the muscles, and every manual 

 effort whether of the artist or the mechanic would be confused and 

 disorderly. The step, too, would be unsteady and insecure. "In 

 chewing our food," s^ys Dr. A. Combe, 1 "in turning the eyes towards 

 an object looked at, in raising the hand to the mouth, and, in fact, in 

 every variety of muscular movement which we perform, we are guided 

 by the muscular sense in proportioning the effect to the resistance to 

 be overcome; and where this harmony is destroyed by disease, the 

 extent of the service rendered us becomes more apparent. The shake 

 of the arm and hand which we see in drunkards, and their consequent 

 incapability of carrying the morsel directly to the mouth, are examples 

 of what would be of daily occurrence, unless we were directed and 

 assisted by a muscular sense." It enables us to form ideas of force and 

 resistance, by conveying to our minds a distinct idea of the effort re- 

 quired. 



The force or intensity of muscular contraction is dependent upon two 

 causes, the physical condition of the muscle, and the energy of the 

 brain. A muscle, which is composed of large, firm fibres, will con- 

 tract, the energy of the brain being equal, more forcibly than one 

 with delicate, loose fibres. Volition generally determines the degree of 

 power developed by the voluntary motions; and is accurately regulated 

 so as to raise a weight of one pound or one hundred. We notice 

 astonishing efforts of strength in those that are labouring, at the time, 

 under strong cerebral excitement; mania, rage, delirium, &c. In such 

 cases, the delicate muscles of the female are capable of contracting 

 with a force far transcending that of the healthy male. The power of 

 muscular contraction is, therefore, in a compound ratio with the 

 strength of the organization of the muscle, and the degree of excitation 

 of the brain. When both are considerable, the feats of strength sur- 

 pass belief; and where both are small, the results are insignificant. 

 The extensors of the knee and foot occasionally contract with so much 

 violence as to fracture the patella and tendo Achillis, respectively. 

 The force, developed in the calf of the leg, must be great, when a per- 

 son stands on tiptoe with a burden on his head or shoulders; or when 

 he projects his body from the soil, as in leaping. Rudolphi 2 asserts, 

 that he has seen a horse, which fractured its under-jaw by biting a 

 piece of iron. 



It has been a question, whether the power of a muscle is greater or 

 less at different degrees of contraction, the same stimulus being applied. 

 To determine this, Schwann 3 invented an apparatus, which should accu- 

 rately measure the length of the muscle, and the weight it would balance 

 by its contraction ; and, from his experiments it appeared, that a uniform 



1 Principles of Physiology, 5th edit., p. 131, Edinb., 1836. 



3 Op. cit., p. 303. 



3 J. Miiller, Physiology, p. 903. 



