Among the conditions necessary for time exercise of the sense of touch are (1) a nor mal condition of the papillary apparatus and of the nerves supplying it; (2) a due sup ply of blood to the tactile organs; and (3), as has been noticed in tlie article TASTE, a tem perature not too far removed from the natural heat of the It has been shown by prof. Weber, that, if the fingers or the lips be immersed for half a minute or a minute in water heated to 125°, or cooled to 32°, the power of distinguishing between a hot or cold fluid or solid body is for the time completely lost, a feeling of pain alone being •experienced. The result was the same on applying cold to the trunk of a nerve, the ulnar nerve at the elbow, where it lies just beneath the skin, being selected for the experiment. The fingers supplied by this nerve soon lost the power of distinguishing between heat and cold, and could only imperfectly perceive the contact and pressure of bodies.
The above-named physiologist has made a large number of experiments on the gen eral subject of touch. His investigations regarding the tactile discrimination in differ ent parts of the skin have been noticed in the aricle SENSIBILITY. Prof. Valentin, whose results, on the whole, correspond very closely with those of Weber, found, however, a considerable extent of individual variation, some persons being able to distinguish the separate compass-points at half or even one-third of the distances required by others.
There is no sense which is so capable of improvement as that of touch. Of this power
of improving the delicacy of touch, says Dr. Carpenter, " we have examples in the case of certain artisans, whose employments require them to cultivate their tactile discrim ination; thus, the female silk-throwsters of Bengal are said to be able to distinguish by the touch alone twenty different degrees of fineness in the unwound cocoons, which are sorted accordingly; and the Indian muslin-weaver contrives by the delicacy of his touch to make the finest cambric in a loom of such simple construction that European fingers could at best propose to make a piece of canvas at it." The highest degree of tactile sensibility is met with in blind persons—a circumstance which is to be attributed for the most part to the concentration of the attention and of the powers of recollection and comparison which are brought to bear upon the mind; and probably to some extent to an increased development of the tactile organs themselves, resulting, as the above-named physiologist suggests, "from that augmented .nutrition which would be the natural consequence of the frequent use of them, and of the increased flow of blood that seems to take place toward any part on which the attention is constantly fixed."—For much interesting information on this subject, the reader is referred to Dr. Kitto's Lost Senses, in which cases, apparently quite authentic, are given of blind persons being able to distinguish colors by the touch.