Fig. 42.— Sense-Organs susceptible to Pressure.
All are formed on essentially the same plan; a fibrous capsule invests a group of epithelial cells amongst which a nerve ramifies. The simplest form is known as a Grandry’s corpuscle-a nerve ending in one or two plates between two or three epithelial cells. These organs are found in great numbers in the bills of aquatic birds. If a duck is watched whilst it is gobbling mud at the margin of a pond, it will be seen to have a remarkable capacity for discriminating between the shells of small snails, which it can crush, and stones, which it needs to drop from its bill. Its bill is also provided with small Pacinian corpuscles ([Fig. 43]). Touch-corpuscles, more elaborate in form than the one figured, are found in the papillæ of the skin of the fingers and elsewhere. They appear to be modified hair-follicles. End-bulbs occur in the conjunctiva and elsewhere, and especially in the peritoneum. Together with Pacinian corpuscles, they are accountable for sensations connected with the distension of the stomach and intestines.
If sensitiveness to pain is investigated by tapping very gently with a needle—or, better, by using a stiff horsehair fixed in a cleft stick, from which it projects about ¼ inch—it will be found that every here and there are spots which are exceedingly sensitive, whilst adjoining them are areas which are moderately sensitive, and between these areas small spots or stretches of skin which do not give the smarting sensation even though the horsehair be pushed until it doubles up.
Fig. 43.— Pacinian Corpuscle.
These organs are especially numerous in the neighbourhood of tendons and ligaments. They are also present beneath the skin of the hands and feet. Their capsules are formed of a great number of concentric lamellæ of connective tissue, enclosing lymph-spaces. Within the capsule is a core of finely granular substance, which also shows a tendency to a lamellar disposition. The structure of these relatively large sense-organs is highly suggestive of sensitiveness to pressure, traction, or rubbing.
Testing now for sensitiveness to cold with a cold blunt metal point, “cold-spots” can be mapped on the skin. If the metal is warmed to about 50° C., “heat-spots” are found. The different kinds of spot are very irregularly distributed. They may coincide, or overlap, or leave blank spaces. Their relative abundance varies. In some regions touch-spots, in others cold-spots, in others heat-spots, are more closely grouped. The tongue and the hand, and especially the tips of the fingers, are most sensitive to touch; but whereas the tongue is also exceedingly sensitive to warmth, the hands are relatively insensitive. Yet, speaking generally, parts especially sensitive to touch are little sensitive to temperature, and vice versa. Sensitiveness to cold is much more widespread than sensitiveness to heat. It is concentrated in the skin covering the abdominal viscera. A cold douche directed between the shoulders is doubtfully felt as cold. There is no doubt whatever about it when it strikes the skin over the stomach.