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Their Structure and Functions Anatomy and Physiology

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The Conditions of Sensation.—The senses are the avenues by means of which information reaches the individual regarding the condition of his own body, and concerning the outward world which surrounds him, and of the manner in which it affects him. Every living being, even the humblest, is not a mere separate exis tence, having a life of its own and independent of everything else ; it is a part of a greater existence ; and its value is estimated by the nature of the relations it bears to that great world of things of which it is but a mere speck. The most elementary living things have no nervous system, no special apparatus for com municating outwardly. They are little masses of irritable jelly-like living material (proto plasm), capable of acting as a whole, and with no part of their substance devoted to special purposes. An. advance in this structure is per ceived when a living thing shows evidence of having one part of its body devoted to the dis charge of one duty and another part to the performance of another duty. The rudiments of a nervous system are found in some of -the lowest animals, where one cell readily affected by certain agencies—contact with foreign ma terial, for example—is situated near the surface of the body and communicates by a slender thread with a cell, capable of contracting, placed deeply in the body. Whenever the cell on the surface is affected sufficiently, the irritation of its substance that results is communicated along the thread to the deep cell, and excites it to contract, so that the body of the organism is moved. One cell is, as it were, on the look-out, and the business of the other one is to act on receiving information. Such a simple arrange ment is sufficient for an elementary organism. But animals higher in the scale are affected in so many different ways by so many different agencies that a further subdivision of labour becomes necessary. One man may be a suffi cient watch on the top of a small fort, but a large town needs a multitude of watchmen, each with his owu particular duty and his owu special post. So in the higher animals and

man certain organs are set apart to give in formation regarding things the body comes in contact with, their hardness, their degree of heat, &c., all that is included under the sense of touch ; another organ is set apart to give information about smell, another to inform re garding taste; another organ has as its busi ness the duty of being on the look-out for light and colour, another for taking knowledge of sounds. The senses are thus the outposts of the mind, disposed along its walls to take note of and report to head-quarters anything that comes within the range of their duty. With out them man could have no knowledge of the outward world and could hold no relations with it. The information they supply acts, in great measure, as the motives and bases of his action. It is not out of place to remark how great, then, is the need of the information being accurate, and of the outposts being properly trained to their work, lest they mislead the mind I There are thus a number of special sensa tions, touch, taste, smell, seeing, hearing, to which is added the muscular sense, by means of which information is supplied regarding out ward things and forces. Besides these, how ever, there are a number of or general sensations, the need of which arises from the highly complicated structure of the higher ani mals. The feature of such higher animals is the multitude of different organs in the body, each performing its own part of the general work required for maintaining the life and vigour of the animal. These organs must all work in harmony, and are in communication with one another. It is necessary that the individual should have some means of knowing whether the harmony is being maintained, and should have some warning if any organ is doing bad or indifferent work. Such information is supplied by common sensations. Thus a feel ing of comfort informs of general a feeling of hunger or thirst informs of the need of certain substances to maintain nourish ment; a sense of discomfort informs of some disturbance, and so on.

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