OCR Text |
Show 18 INTRODUCTION. the very object of this difference appears to be the securing of this independence. . The nervous functions ' that is, sensibility and m. usc.u lar Ir-ritability, are so much the stronger at every pomt, ~n pro-portion as their exciting cause is abundant! an~ as this cause or the nervous fluid is produced by secretion, Its abundance must be in proportion to the quantity of medullary or secretory matter and the amount of blood received by the latter. In animais that have a circulating system, the blood is propelled through the arteries which convey it to its destined parts, by means of their irritability and that of the heart. If these arteries be irritated, they act more strongly, and propel a greater quantity of blood; the ner~o~~ fluid ~ec~m.es more abundant and augments the local sensibility; this, Ill Its turn augments the irritability of the arteries, so that this mu- tual 'a ction may sometimes be carried to a great extent. I t 1.s called orgasm, and when it becomes painful and permanent, inflammation. The irritation may also originate in the nerve when exposed to the influence of acute sensations. . . This mutual influence of the nerves and fibres, either m· testinal or arterial, is the real spring of vegetative life in ani-mals. As each external sense is permeable only by such or such sensible substances, so each internal organ may be accessible only to this or that agent of irritation. Thus, mercury irritates the salivary glands, cantharides irritate the bladder, &c. These agents are called specifics. The ·nervous system being homogeneous and continuous, local sensations and irritation debilitate the whole, and each function, by excessive action, may weaken the others. Excess of aliment weakens the power of thought, while long continued meditation impairs that of digestion, &c. Excessive local irritation will enfeeble the whole body, as if all the powers of life were concentrated in one single point. A second irritation produced at another part may diminish, or divert, as it is termed, the first: such is the effect of blis· ters, pu1·gatives, &c. . Brief as our sketch has been, it is sufficient to establish the t : ... -·· INTRODUCTION. 19 possibility of accounting for all the phenomena of physical life from the properties it presents, by the simple admission of ~ fluid such as we have defined. Summary idea of the Functions and Organs of the Bodies of .llnimals, and of their various degrees of Complication. After what we have stated respecting the organic elements of the body, its chemical principles and acting powers, nothing remains but to give a summary idea of the functions of which life is composed, and of their appropriate organs. The functions of the animal body are divided into two classes: The animal functions, or those proper to animals, that is to say, sensibility and voluntary motion. The vital, vegetative functions, or those common to animals and vegetables, i. e., nutrition and generation. Sensibility resides in the nervous system. The most general external sense is that of touch; it is seated in the skin, a membrane that envelopes the whole body, which is traversed in every direction by nerves whose extreme filaments expand on the surface into papillre, and are protected by the epidermis and other insensible teguments, such as hairs, scales, &c. &c. Taste and smell are merely delicate states of the sense of touch, for which the skin of the mouth and nostrils is particularly organized: the first, by means of papillre more convex and spongy; the second, by its extreme delicacy and the multiplication of its ever humid surface. We have alr~ady. spoken of the ear and the eye. The organ of generation IS endowed with a sixth sense, seated in its internal skin; that of the stomach and intestines declares the state of those viscera by peculiar sensations. In fine, sensations more or less painful may originate in every part of the body through accident o:r disease. . Many animals have neither ears nor nostrils, several are Without eyes, and some are reduced to the single sense of touch, which is never absent. |