From these observations it appears that the skin contains three sets of organs sensitive respectively to touch, cold, and heat. Certain investigators hold that it also contains specific organs, or nerve-endings, sensitive to painful stimulants; but in this case there is the obvious difficulty of distinguishing between pain and touch. At no spot can pure pain be evoked free from any consciousness of touch.
To a certain extent the combinations of epithelial cells and nerve-endings in the skin fulfil the negative requirement of sense-organs; each kind, whilst specially sensitive to its own specific stimulant, is insensitive to stimulants of other kinds. But mutual exclusion is not absolute in the case of cold and warmth. If a warmed metal point be applied to a cold spot, it produces a sensation of cold. Our feelings of warmth and cold are to a large degree comparative. Luke-warm water feels cold to hands just taken out of hot water; moderately cold water appears luke-warm to hands that have been in contact with ice. The sensory apparatus for cold and heat soon adapts itself, or, in physiological language, it is soon fatigued. If after a prolonged bath at the body temperature a foot be plunged into very hot water and withdrawn quickly, the feeling which first ensues is one of cold. It is indistinguishable from the feeling provoked by dipping the foot into cold water. The sensation of cold subsequently gives place to one of painful warmth. This does not indicate that the heat-spots have been waked out of their lethargy by excessive stimulation. On the contrary, it is the cold-spots which, when they were first stimulated by the very hot water, answered “Cold,” that now cry out “Hot”; for both cold-spots and heat-spots, when strongly stimulated, yield the same sensation. Indeed, it appears that the mind relies upon the simultaneous stimulation of adjacent heat-spots and cold-spots for the assurance that the thing with which the skin is in contact is really hot. If two metal points, one kept warm and the other cold, are applied simultaneously to two closely adjacent spots of skin, the resulting sensation is “hot.” When the cold point is withdrawn, or replaced by a second warm point, the sensation sinks to “warm.”
CHAPTER XVI
VOICE AND SPEECH
A cut carried horizontally backwards across the cartilage which projects forwards as Adam’s apple, a quarter of an inch below its notch, would show that it is V-shaped, the point of the V in front. Each limb of the V is a broad plate. In the mid-line is a gap, the rima glottidis, through which the windpipe communicates with the pharynx ([Fig. 45]). It is overhung by the stiff leaf-shaped epiglottis, the edge of which can be felt with the finger behind the tongue. (γλωττίς, the mouthpiece of a reed-pipe, is the term commonly used, for short, for the rima glottidis.) When air is being drawn into the lungs, the glottis is widely open. In speaking or singing it is almost closed. It is tightly shut whilst food is passing down the gullet.
The glottis is bounded, as to its anterior two-thirds, by two membranous folds, the vocal cords. In its posterior third it has a triangular cartilage, the arytenoid, on either side. A distinction is sometimes drawn between the anterior part, bounded by the vocal cords, and the whole glottis, the former being termed “rima vocalis”; but it is scarcely justified, for, although it is true that the anterior part is essentially the organ of voice, and its margins alone vibrate when high notes are sung, the anterior ends of the arytenoid cartilages also vibrate during the production of low notes. (The substance of these processes is not, properly speaking, cartilage; it resembles the epiglottis in containing a great abundance of elastic fibres.) And here we must warn the reader not to picture to himself a vocal “cord” as a kind of fiddle-string. It bears no resemblance to a cord, as we ordinarily understand the word; it is but a fold of mucous membrane, such as one might pinch up between finger and thumb from the inner side of the cheek. Its capacity for vibration depends upon the tenseness which is given to it by the pressure of the lymph with which it is distended, and vast numbers of exceedingly slender elastic fibres which traverse it.
Fig. 44.—The Anterior Half of the Larynx seen from Behind.