206 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



in the greatest amount in certain forms of the non-striated fibre, is called 

 Tonidty. 



348. That the Irritability of Muscles is a property inherent in them, 

 and is in this respect analogous to the peculiar vital endowments of any 

 other form of tissue, cannot be any longer a matter of doubt ; though 

 many Physiologists have sought to show, that it is in some way derived 

 from the nerves. Not only may an entire Muscle be made to contract, 

 by the application of a proper stimulus, long after the division of the 

 nervous trunks supplying it ; but even a single fibre, completely isolated 

 from all its nervous connexions, may be seen to contract under the 

 Microscope. Moreover, in the non-striated muscular fibre, it is often 

 difficult to excite contractions through the nerves at all, when a stimulus 

 directly applied to itself will immediately produce sensible and vigorous 

 movements. The energy of the contractile power depends in great part 

 upon the state of nutrition of the muscle ; and this again is influenced 

 by the degree in which it is exercised. NQW as the Muscles of Animal 

 Life are all excited to action, in the usual state of things, through the 

 medium of their nerves, it follows that if the nerves be paralysed, the 

 muscles will be seldom or never called into use. When disused, they 

 will receive very little nourishment ; the disintegrating changes will not 

 be counterbalanced by reparative processes ; and in consequence, the 

 muscular structure will be gradually so far impaired, as to lose its pecu- 

 liar properties, and will even, in time, almost totally disappear. Yet 

 even after the almost complete departure of muscular contractility, 

 through the metamorphosis of the structure consequent upon disuse, it 

 may be again recovered, if the muscles be called into exercise ; but the 

 recovery of the power is very slow, and proceeds pari passu with the 

 improvement in the nutrition of the part, being more tedious in propor- 

 tion to the length of the previous disuse. 



349. That the Irritability of Muscular fibre belongs to itself, and is 

 not derived in any way from the nerves, is further shown in the fol- 

 lowing manner. If a set of muscles (as those of the leg of a Rabbit 

 or Frog) be repeatedly thrown into action by galvanism, until the 

 stimulus will no longer occasion their contraction, their irritability is 

 then said to be exhausted ; by rest, however, it is recovered, the nu- 

 tritive processes making good the loss previously suffered. Now it has 

 been shown by Dr. J. Reid, that this recovery may take place, even 

 after the division of all the nerves supplying the limb ; provided that 

 the nutrition of the part be not interfered with. It has been further 

 shown by the same excellent Physiologist, that, if the nerves of a limb 

 be divided, the loss or retention of the contractility of the muscles en- 

 tirely depends upon the degree of exercise to which they are subjected, 

 and consequently upon the nutrition they receive. The muscles of the 

 hind-leg of a Rabbit, whose sciatic nerve had been divided, were found 

 to lose their contractility almost completely in the course of seven 

 weeks. They were much smaller, paler and softer, than the corre- 

 sponding muscles of the opposite leg ; and they scarcely weighed more 

 than half as much as the latter. Now when the nerves of both hind- 

 legs of a Frog were cut, and the muscles of one of the limbs thus 

 paralysed were daily exercised by a weak galvanic battery, while those 



