448 OF MUSCULAR CONTRACTION. 



servation of Mr. Bowman already cited ( 231), that a single fibre, completely isolated from 

 all its connections, may be seen with the microscope to pass into a state of contraction, under 

 the influence of direct irritation. Further, it has been experimentally ascertained, that there 

 are some chemical stimuli, which will produce the contraction of muscles when directly ap- 

 plied to them, but of which the influence cannot be transmitted through the nerves ; this is 

 especially the case with regard to acids. 



591. When all these considerations are allowed their due weight, we can 

 scarcely do otherwise than acquiesce fully in the doctrine of Haller, which 

 involves no hypothesis, and which is perfectly conformable to the analogy of 

 other departments of Physiology. He regarded every part of the body which 

 is endowed with Irritability, as possessing that property in and by itself; but 

 considered that the property is subjected to excitement and control from the 

 Nervous System, the agency of which is one of the stimuli that can call it 

 into operation. It may be desirable briefly to recapitulate the facts, by which 

 this doctrine is supported. 1. The existence in Vegetables of irritable tissues, 

 which are excited to contraction by stimuli directly applied to themselves, 

 and which can be in no way dependent upon, or influenced by, a Nervous 

 system. 2. The existence in Animals of a form of Muscular tissue, which 

 is especially connected with the maintenance of the Organic functions, and 

 which is much more readily excited to action by direct stimulation, than it is 

 by Nervous agency. 3. The fact that, by the agency of these, the Organic 

 functions may go on (as long as their other requisite conditions are supplied) 

 after the removal of the nervous centres, and when none were ever present ; 

 rendering it next to certain, that their ordinary operations are not dependent 

 upon any stimuli received through the nerves, but upon those directly applied 

 to themselves. 4. The persistence of irritability in muscles, for some time 

 after the nerves have ceased to be able to convey them the effects of stimuli; 

 this is constantly seen in regard to the Sympathetic system of nerves, and 

 the muscles of Organic life upon which they operate; and it may also be 

 shown to occur with respect to the Cerebro-Spinal system, and the muscles 

 of Animal life, by the agency of narcotics. 5. The persistence of irritability 

 in the muscles, after their complete isolation from the nervous centres, so 

 long as their nutrition is unimpaired; and the effects of frequent exercise, in 

 preventing the impairment of the nutrition and the loss of irritability. 6. 

 The recovery of the irritability of muscles, when isolated from the nervous 

 centres, after it has been exhausted by repeated stimulation; this also depends 

 upon the healthy performance of the nutritive actions. 7. The contraction 

 of muscular fibre under the microscope, when completely isolated from .all 

 other tissues. In the words of Dr. Alison, then, "the only ascertained final 

 cause of all endowments bestowed on Nerves in relation to Muscles, in the 

 living body, appears to be, not to make Muscles irritable, but to subject their 

 irritability, in different ways, to the dominion of the acts and feelings of the 

 Mind," to its volitions, emotions, and instinctive determinations. 



592. A curious question has been lately raised, the decision on which is of 

 some importance in our determination of the nature of the force, by which 

 the contraction of muscles is occasioned. This is, whether the power of a 

 muscle is greater or less at different degrees of contraction, the same stimulus 

 being applied. This seems to have been determined, by the ingeniously-de- 

 vised experiments of Schwann.* He contrived an apparatus, which should 

 accurately measure the length of the muscle, and, at the same time, the weight 

 which it would balance by its contraction. Having caused the muscle of a 

 Frog to shorten to its extreme point, by the stimulus of galvanism applied to 

 the nerve, so that no further stimulation could lift a weight placed in the oppo- 

 site scale, he allowed the muscle to relax until it was extended to a certain 



* fuller's Physiology, p. 



