326 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



difficulty that these can be made to display their irritability, by any stimuli 

 applied to their nerves ; whilst they manifest it strongly, when the stimulus is 

 directly applied to themselves. Even in the Muscles of Animal life, individual 

 fasciculi may be thrown into action in the same manner; although the entire 

 mass cannot be put into combined operation, except by a stimulus simultaneously 

 communicated to the whole, which the nerve affords the readiest means of effect- 

 ing. Perhaps the most satisfactory disproof of it, however, is to be found in 

 the observation of Mr. Bowman already, cited ( 302), that a single fibre, com- 

 pletely 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 applied to them, 

 but of which the influence cannot be transmitted through the nerves; this is 

 especially the case with regard to acids. 



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

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

 volves 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 con- 

 sidered that the property is subjected to excitement and control from the Nerv- 

 ous 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 (so long as their other requisite conditions are supplied) after the removal 

 of the nervous centres (of the Cerebro-spinal system at least) and when these 

 were never 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 to 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 is 

 shown, by the agency of narcotics, to be true also with respect to the Cerebro- 

 Spinal system and the muscles of Animal life. 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 pre- 

 venting 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 endow- 

 ments 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. 



328. Whilst the Irritability of Muscles is gradually departing after death, it 

 not unfrequently shows itself under a peculiar form ; for instead of producing sud- 

 den contractions, speedily followed by relaxation, the application of stimuli then 

 occasions slow and somewhat prolonged contractions, the relaxation after which 



