The nerves which minister to specia sensation, differ from the nerves of common sensation in no essential point of their ana tomy, except in their mode of organisation at the periphery of the body. Each of them has, probably, likewise some peculiarity of connection with the brain : this is obvious as regards the olfactory and the optic nerves ; less so as regards the nerves of taste, touch, and hearing. The physiological peculiarity of these nerves is then, in all probability, due to their central and peripheral organisation ; and especially, perhaps, to the latter, which, doubt less, renders them peculiarly susceptible of the influence of those delicate physical agencies to which each of them is exposed.
The nerves and organs of special sensa tion, especially those of touch, are so com prehensive in their objects, that it would almost seem that little was left for the so called nerves of common sensation.
These latter nerves, nevertheless, serve many important objects ; they doubtless ex cite in the mind many feelings, agreeable or disagreeable, of pain or of pleasure, or even feelings neutral as regards pain and pleasure, which could not be developed through the nerves of special sense. The consciousness of the integrity of our limbs and of the general framework of our bodies, is secured, in a great measure, through the instrumentality- of these nerves. Injuries to various parts—disturbances in their nutrition, as inflammations, ulcera tions, &c.— are made known to the mind by the painful sensation excited through these nerves. The sensibility of organs and textures
— e. the degree to which affections of these parts are capable of inducing corresponding affections of the mind — depends upon the number of these nerves which are distributed to them — the degree of sensibility being in proportion to the number of the nerves. Hence these nerves of common sensation ex ercise a conservative influence over the several textures and organs to which they are dis tributed, and serve to afford warning of the approach or of the existence of danger.
What some have called the muscular sense, e. the knowledge which we have of the state of our muscles, is generally attributed to these same nerves. As the sensibility of the muscles is doubtless due to these nerves, we may reasonably impute to them the faculty of informing the mind of the state and degree of contraction or relaxation of the muscles, and thus of contributing to that power of adjust ment which is necessary to give precision to our muscular efforts. This sense comes greatly in aid of that of touch, and of those powers which we derive from the sense of touch.
It admits of question whether this sense really requires the presence of true nerves of sensation in the muscles, and whether it may not be due to the reaction of the muscular force upon the proper muscular or motor nerves, through which, by reflection at the centre, the centre of sensation becomes affected. (See