Innumerable facts are capable of being adduced, to prove that sensation is a cause of muscular action. There is, however, little necessity to be tedious with the proof; because there will be little difficulty in assenting to the proposition.
The distinction, which we formerly drew, between those sensations which we have by what is called the external senses, in other words, on the surface of our body, and those (numerous, not individually only, but also in their species or kinds), which we have in 331 the internal parts of our bodies, it is here peculiarly necessary to remember, and strongly to remark. The muscles themselves are internal parts of the body. The feelings in the muscles are one species of those internal sensations. And, in general, as it is easy to conceive, the internal sensations are a leading cause of such actions as take place in the internal organs of the Body.[59]
[59] The actions which take place in the interior of the body are not always, nor perhaps even generally, produced by sensations. A large portion of them are not preceded by any sensation of which we are aware, and have been ascertained to depend on nerves not terminating in the brain, which is the seat of sensation, but stopping at the spinal cord. These actions are inferred to be the results of a mere physical stimulus, operating either upon the local nerves, or upon the spinal ganglions with which those nerves communicate, and not attended with any consciousness.
Many of the instances which the author goes on to enumerate, of muscular action excited by sensation, are, in all probability, cases of this description. The muscular action is directly excited by the physical irritation of the nerves, and any sensation which accompanies it is not its cause, but a simultaneous effect.—Ed.
Some of the external cases are remarkably familiar and precise. A pungent odour enters the nostrils; first, a certain sensation follows, and immediately after, the violent action of a great number of muscles, called Sneezing. In drinking, a drop of water sometimes enters the larynx; it produces a certain sensation, immediately followed by the action of certain muscles, from which we have the very painful feeling of suffocation. There is a very remarkable exemplification of the same law, in the case of the sensation 332 of light. The Pupil of the Eye contracts or dilates, according as a greater or less degree of light falls upon the retina. The eyelids are in perpetual motion in consequence of sensations to which we do not attend. The painful sensation pervading the body, when we plunge into cold water, produces so much action in the muscles, that we sob and respire in a convulsive manner. The lachrymal glands are moved to action, by certain effluvia, as those of onions, by smoke, and various gases, and even by certain states of the air, so as to shed tears abundantly. The action of food is similar upon the salivary glands; and of heat and cold upon the skin, the one opening, the other contracting its pores.
In respect to a great number of the contractions of muscles, which take place in consequence of impressions on the surface of our bodies, the evidence is not so precise; because, though contractions are originally performed by sensation, they are afterwards and more habitually performed by Ideas. We shall be able, therefore, to speak of them more instructively, when more familiar with the sequence consisting of Ideas antecedent, and the contraction of muscles consequent.
The action of the internal organs in consequence of internal sensations, is proved by many familiar, as well as by many interesting phenomena. The action of coughing, than which none more familiar, is the highest evidence. The sensation here, is not one of those which are neglected and obscure. A violent action of the muscles is its immediate consequence. Hiccup is also produced by a sensation in the stomach; and affords evidence definite and decisive. Vomiting is another very instructive case. We 333 know that it is the ultimate effect of something which produces disagreeable sensations in the stomach. The sensation, indeed, in this case, is not so well distinguished from others, nor so precisely known, as in the case of coughing. We know, however, its general character, and we know well the violent contraction of muscles, which is the consequence of it. In connexion with this, we may notice the peculiar sensations in the Uterus, which produce the muscular actions of Parturition; some of the most violent belonging to the human frame. The sensations, which are the cause of cramps, are commonly obscure. It is the Effect which engages all our attention. There is no doubt, however, that it is by an internal sensation, that this very painful effect is produced. A greater proportion of those painful muscular actions called spasms, are the effect of sensations; though Ideas, also, appear to be concerned in the production of those which become frequent. One very remarkable case, which is named the Locked Jaw, is often the result of a pain produced by an external wound.
Not any of our bodily functions is more important than Respiration. It is a very extensive action of muscles habitually performed by sensation merely. The sensations, however, escape our attention to such a degree, that we lose the power of attending to them. And it is only by the effort we are capable of to stop Respiration, when a painful sensation after a time renders the action of the muscles irresistible, that we get a sort of conjectural knowledge of what the ordinary sensation is.
There are some most important cases of the action of our internal organs, in consequence of sensation, in 334 which, from the habitual neglect of that which never calls for our attention, both cause and effect, to our ordinary perception, are alike unknown. That the heart is a part of the body endowed with sensation, is abundantly known, as often as, by a departure from its habitual state, it becomes the seat of sensations other than the habitual sensations, to which, from habit of inattention to them, we have lost the power of attending. The blood cannot flow into the heart, without a sensation of the heart. The contraction of the heart is the consequence of that sensation; thence the circulation of the blood; thence respiration, and all the trains, both of sensations, and of actions, which constitute the general working of the human machine. In truth, the actions of the alimentary canal, necessary to keep up the supply of the blood and the actions of the circulating system, which impart their action to most of the assimilating and secreting organs of the human body, all taking place in sensitive parts, all, of course, attended by sensation, and all produced by sensation, constitute a system of internal sensations, numerous beyond what it is easy to conceive,—some pleasurable, some painful,—and of all possible modifications of pain and pleasure; but to which, singly, the habit of inattention is so complete, that it amounts to inability of attending to them.
When they are very extensively of a pleasurable, or very extensively of a painful kind, they produce a general state, which often calls our attention; but for which, as it is a vague, indeterminate feeling, we have only vague, indeterminate names: we call it a 335 state of comfort or discomfort; of cheerfulness, or gloom; high spirits, or low spirits; and so on. The incessant motion of the blood, in so many sensitive tubes, in every part of the body, constitutes a system of sensations pervading the whole frame; as the contact of the air produces a system of sensations, pervading every part of the surface of our bodies, but to which our habit of inattention is so complete, that we are equally incapable of attending to them as we are of attending to the sensations produced in our arteries and veins, by the motion of the blood, and in the secreting and absorbing vessels when excited to action.[10*]