438 OF MUSCULAR CONTRACTION. 



guided in their action by the sensations received through the Ears, in the same 

 manner as other muscles are guided by the sensations received through them- 

 selves ( 433). On this point, more will be said hereafter (611). 



CHAPTER VII. 



OF MUSCULAR ACTION. 



1. Of Contractility in General. 



573. THE Nervous System has no power of occasioning movement in any 

 part of the body, save by exciting to contraction certain structures, to which 

 the term Muscular is given. That one tissue should possess within itself the 

 property of Contractility on the application of a stimulus, is no more wonderful, 

 than that another should be capable of conveying sensory or motor influences, 

 or another of separating a peculiar secretion from the blood. Such contractile 

 tissues are found in Vegetables, as well as in Animals ; and they appear to 

 consist, in both instances, of cells, whose peculiar property it is to change 

 their form, when subjected to certain kinds of irritation ( 230). The only 

 essential difference in function, between the Contractility of the cells compos- 

 ing the ultimate fibrillre of Muscular Fibre, and that of the cells composing 

 the intumescence of the Sensitive Plant, consists in the susceptibility of the 

 former to a stimulus, which does not operate on the latter. Both can be made 

 to change their form by stimuli of various kinds, mechanical, chemical, 

 electrical, &c., directly applied to themselves ; but the contractility of Mus- 

 cular fibre is excited, in addition, by the stimulus of Innervation, which has 

 no operation in the Plant ; and it is when its peculiar property is thus excited, 

 that the Muscular tissue becomes the instrument of the operation of the Nerv- 

 ous system upon the external world, and thus performs an important part in 

 the purely Animal Functions. The Muscular tissue, however, is not always 

 thus called into activity through the medium of the Nervous system ; for it is 

 employed to execute numerous movements, which are immediately connected 

 with the maintenance of the Organic functions, and in which the influence of 

 Innervation seems to be but little concerned; its contractility being excited to 

 action by stimuli directly applied to itself. We have seen that there are two 

 forms of Muscular tissue, the striated and the non-striated, which are appro- 

 priated to these two purposes ; the former being the kind most readily acted 

 on through the Nervous System, and invariably employed in the Muscles that 

 are called into action by its influence ; whilst the latter (which seems a less 

 perfectly-developed form of the tissue) is with difficulty excited to contraction 

 through the Nervous System, and is usually employed in Muscles, whose 

 action is altogether uncontrollable by the will ( 225 234). 



574. The general property of Contractility shows itself under two forms ; 

 which are alike distinct in the mode of their action, and in the conditions 

 requisite for its excitation. Its most obvious and striking manifestations pre- 

 sent themselves in the Voluntary muscles and in the Heart ; which, when in 

 activity, exhibit powerful contractions tending to alternate with relaxations. 

 The modification of contractility which is concerned in producing these, is 

 distinguished as Irritability. On the other hand, we find that the muscles 



