This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
1. There are various properties which distinguish living from dead animal matter. Both, in common, possess certain physical properties, such as weight, extensibility, flexibility, etc. ; but living matter has certain physiological or vital properties peculiar to it. It is these which bring it under the influence of external agents.
2. All the phenomena of life are the effect of impressions made upon the various organs of the body, by external or internal agents; and each organ has its own proper stimulus. Thus the eye is stimulated by light; the ear by sound, the nose by odours, the organs of taste by condiments, etc.
3. The chick in its shell is developed by the influence of heat; the seed germinates under the combined effect of warmth and moisture ; and so the growth of the human body requires the application of the appropriate stimulants, such as food, drink, air, exercise, etc.
4. Every tissue and organ has its own mode of activity. For example, the lungs react under the influence of the air; the heart under that of the blood ; the muscles under that of the will; the stomach under that of food, etc. Every gland, though supplied by the same blood, is excited to secrete or form that particular fluid for which it was so designed ; and why the liver does not secrete urine and the kidneys bile, it were useless to inquire.
5. This property of living matter has three principal modifications in the solids and fluids, which have been called sensitive, motive, and alterative. By the sensitive powers, are meant sensibility and its modifications ; the motive are contractility and expansibility; the alterative are those which preside over the formation and nutrition of the different organs and tissues of the body.
6. Sensibility belongs exclusively to animals provided with a nervous system. It enables us to receive impressions from external objects, or from changes going on in our own bodies ; and to the accuracy of this power, we owe our ability to guard ourselves against the influence of noxious agents.
7. Sensibility may be divided into two kinds, general and special. By general sensibility is meant, that universal sense, of which we are conscious over the whole body, in the mouth, etc. The same also exists in the interior of the body, and conveys- to the mind a knowledge of the wants of the system ; and when disease attacks, any part, it immediately apprizes us of the danger, in order that we may take early measures to remove it. By special sensibility, we mean that property which renders the eye sensible to light, the ear to sound, etc. Every organ has its own special sensibility.
8. The brain is the common centre of sensibility, both general and special. No impression can be felt on any part or organ, unless it has a connection with the brain. If the nerves going from that organ are divided, or if their function is lost from palsy or any cause whatever, there can be no sensation perceived, as none is excited.
9. By perception is meant, the faculty which the brain has of perceiving or taking notice of these impressions. Perception and thought then, are owing to the property of matter, called sensibility. Without it, we would be like stocks or stones,, alike unconscious of pleasure or of pain.
10. According to some physiologists, there is another kind of sensibility, which is termed organic. This resides in the several internal organs, where it is called into exercise, and does not require the action of the brain. It depends for nervous influence upon the great sympathetic nerve. That such a kind of sensibility exists is very probable, but we have no proof of its existence in our own feelings and consciousness, as we have of the other kinds of sensibility.
11. By organic sensibility, we mean that the stomach is sensible to food, the heart to the blood, etc. ; and that this feeling is confined to the organ and not transmitted to the brain. It presides over the process of digestion, circulation, secretion, absorption, and nutrition.
12. Although the internal organs of the body are not sensible to the presence of the fluids or solids with which they are usually in contact, yet if foreign bodies are brought in contact with them, or substances calculated to injure them, we are immediately made sensible of it. Thus let a person drink a quantity of brandy, or spirits, to which he is not accustomed, and he will at once feel a sensation of heat in the region of the stomach, altogether unnatural to that organ. This proves that ardent spirits are not designed for the drink of man, and are therefore hurtful.
13. Certain parts of the body, which, in a healthy state, are nearly insensible ; yet, by disease, become the seats of acute pain. This is particularly the case with the bones, cartilages, and ligaments-parts usually wholly destitute of feeling.
14. Contractility, or the property of contracting, is the chief motive power of the system. It exists in various degrees, in different kinds of animal matter. That element which possesses it in the greatest degree is fibrin; and those tissues which have the most fibrin, have the greatest degree of contractility. The same is true of muscles, for the heart, which is in constant motion, is almost pure fibrin.
15. It is supposed that the coagulation of the blood is owing to this contractile power of fibrin. In the living vessels, the blood is kept fluid by the vital influence of the walls of the vessels themselves ; but as soon as it is withdrawn from this influence, the particles of fibrin immediately rush together and form a solid mass.
16. Those tissues which contain but little fibrin and are made up mostly of gelatine, as the membranes, cartilages, skin, vessels, etc, have but a slight degree of contractility ; that they possess it, in some measure, is evident from the contraction of blood vessels by the application of stimulants.
17. There are two modifications of contractility ; one of which depends for its exercise on the brain, and the other does not. For example, if we wish to bend the arm, we have but to transmit to the muscles of the arm, through the nerves with which it is supplied from the brain, a volition, or act of the will, to that effect, and the arm bends. Here we perceive that the influence of the brain is necessary to contractility or motion.