_Number of the Senses._--The different senses are usually reckoned as five in number. They may all be regarded, however, as modifications of one general sense, that of touch--or, in other words, the susceptibility of the nervous system to be excited by foreign substances brought into contact with it. This is the essential condition of sensation in any case, and the several senses, so called, are but so many variations in the mode of manifesting this excitability. There is a reason, nevertheless, for assigning five of these modifications and no more, and that is, that the anatomical structure indicates either a distinct organ, as the ear, the eye, etc., or at least a distinct branch of the nervous apparatus, as in the case of smell and taste, while the whole nervous expansion as spread out over the surface of the body contributes to the general sense of touch. _The Senses related to each other.--Distinct Office of each._--It is evident enough that these several senses sustain a certain relation to each other. They are so many and no more, not merely by accident; not merely because so many could find room in the bodily organization; not merely because it might be convenient to have so many. Let us look at the office performed by each, and we shall see that while each has its distinct function, not interchangeable with that of any other, it is a function more or less necessary to the animal economy. Remembering that the design and use of the several senses is to put us in possession of data, by means of which, directly or indirectly, we may gain correct knowledge of the external world, let us suppose the inquiry to be raised, What senses ought man to have for this purpose? What does he need, the material universe remaining what it is? _Function of the Sense of Touch._--Things exist about us in space, having certain properties and relations. We need a sense then, first and chiefly, that shall acquaint us with objects thus existing, taking cognizance of what lies immediately about us in space. This we have in the general sense of touch, making us acquainted with certain objective or mechanical qualities of external objects. _This Sense, how limited._--This, however, avails only for objects within a short distance, and capable of being brought into contact. It operates also synthetically and slowly, part after part of the object being given as we are brought into contact with different portions of it successively until the process is so far complete that, from the ensemble of these different parts, our understanding can construct the whole.