When the eyes are closed and a point of the skin is touched we can with some accuracy indicate the region stimulated; although tactile feelings are alike in general characters, they differ in something (local sign) besides intensity by which we can distinguish them as originated on different parts of the skin. The accuracy of the localizing power varies widely in different skin regions, and is measured by observing the least distance which must separate two objects (as the blunted points of a pair of compasses) in order that they may be felt as two. The following table illustrates some of the differences observed:

What sensations do we get through the skin? Name another part of the body which, also gives rise to these sensations. What do we recognize by means of the sense of touch? Where is the tactile sense most acute?

What is meant by the localization of skin sensations? Does the accuracy of localization differ in different regions of the skin? Illustrate.

The Temperature Senses

By these is meant our faculty of perceiving cold and warmth; and, with the help of these sensations, of perceiving temperature differences in external objects. The organs are the skin, the mucous membrane of mouth, pharynx, and gullet, and of the entry of the nose. Burning the skin will cause pain, but not a true temperature sensation, which is quite as different from pain as is touch.

Smell

The olfactory organ consists of the mucous membrane of the upper parts of the nasal cavities; in it the endings of the olfactory nerves are spread. It covers the upper and lower turbinate bones (o, p, Fig. 41) (which are expansions of the ethmoid on the outer wall of the nostril-chamber), the opposite part of the partition between the nares, and that part of the roof of the nose (n, Fig. 41) which separates it from the cranial cavity.

Odorous Substances

Odorous Substances, the stimuli of the olfactory apparatus, are always gaseous. They frequently act powerfully when present in very small quantity. A grain or two of musk kept in a room will give the air in it an odor for years, and yet at the end will hardly have diminished in weight, so infinitesimal is the quantity given off from it to the air and able to excite the sense of smell. While some gases or vapors have this powerful influence upon the olfactory organ, others, as pure air, do not stimulate it at all.

What is the temperature sense? What are its organs? Of what does the olfactory organ consist?

In what point do all odorous substances agree? Illustrate the efficiency, so far as producing smell sensations is concerned, of a very small quantity of an odorous substance. Do all gases stimulate the olfactory apparatus?