By these organs we become acquainted with what passes around us; but the nervous system gives us notice of many changes which take place within our bodies. Internal pains point out to us the presence and situation of diseases; and the disagreeable sensations of hunger, thirst, and fatigue, incline us to give refreshment and repose to the body. It is also by means of the nervous system, that we experience the passions and emotions of the mind.
There are some animals so simple in their structure, that neither brain, nor organs of sense have been detected; yet they are endowed with motion, and are capable of selecting and swallowing their food, and expelling their excrement; and as these acts appear to be voluntary, we must conclude, that they possess nervous matter, though it be so interwoven with the rest of their structure that we cannot exhibit or detect it.
All these different structures which have been described as entering into the formation of a perfect animal, are soft and flexible in themselves, and, in order to the right performance of their functions, require the support of a substance of considerable firmness, which may preserve them in their relative situations, and give a general shape to the body. For this purpose, bones are formed in the higher orders of animals. They consist of a certain portion of animal matter, on which their powers of life depend, mixed with a portion of earthy matter, which gives them a degree of solidity. The firmest substance in the body, composed entirely of animal matter, is cartilage, which possesses, however, too little solidity for the support of animals of considerable size, living in so rare a medium as air. Hence it happens that when the earthy part is, by disease, abstracted from the bones, they become bent and deformed by the weight of the body, or the action of its moving powers. In fishes, however, who inhabit a denser medium, cartilage becomes a convenient structure, being sufficiently firm for their support, and, from its lightness, better suited to their condition.
Had the osseous system been merely intended to give shape to animals, and preserve the relative position of their parts, it might, for any useful purpose, have been as well formed of one piece; and accordingly, when almost all the bones of the body have been anchylosed, or immoveably united to each other by disease, the functions of life have gone on uniformly to an advanced age. There is a remarkable skeleton of this kind preserved at Trinity College, Dublin; where all the large bones of the body are immoveably united together, except the lower jaw, and the joints of the fingers; every joint in the body was immoveable, and yet this person lived to an old age. In order, however, that animals may enjoy a power of changing their situation, the osseous system has been composed of a variety of pieces, and an apparatus added by which this may be easily effected. This is accomplished by adapting the ends of bones to each other so as to form joints, which vary in different parts of the body according to the motion of the part, some being formed for strength, others for extent and variety of motion; the two being incompatible, and never found in the same joint.
In the formation of a joint, however, it appears that two surfaces of bone would move with considerable attrition upon each other, not being capable of a sufficient degree of smoothness; it is therefore necessary, in order to diminish attrition, that a substance be interposed having a high degree of polish; this is supplied by cartilage, with which the ends of all bones, performing motion, are covered; and as animals, both from the common occurrences of life, and from accident, are liable to considerable shocks, in order to guard the system, as much as possible, against injury from these sources, cartilages are endowed with a considerable degree of elasticity, and thus by their reaction are capable of evading certain degrees of violence.
The smoothness of cartilage, however, only prevents attrition to a certain degree; that joints therefore may move with all possible ease and freedom, a fluid is interposed called sinovia. This is separated from the blood, by the vessels distributed to the inner surface of the joint, and is the most slippery of all fluids.
In order that bones may not be separated from each other, but preserve their relative situations, with a certain capacity of motion, it is requisite that they should be joined together; this is done by the ligaments surrounding a joint, which are of two kinds. The one adapted to the firm junction of the bones with each other, upon which the strength of the joint depends; the other loosely attached round the ends of contiguous bones, to secrete sinovia, and retain it in its proper situation; and hence called capsular or purse-like ligament.
This kind of structure, endowed with a power of secreting sinovia, is not confined to the joints alone; for in many parts of the body, where muscles during their action rub on bones, or tendon on tendon, small bags are formed for supplying sinovia, which are called bursæ mucosæ.
As all these parts subservient to motion are inert in themselves, that animals may enjoy the means of changing their situations and attitudes, a power must be applied to the bones for this purpose, which is supplied by muscular action. Thus we find the bones clothed with muscles, which give, in a great measure, the external shape to the body, and act in considerable numbers on the joints, particularly those which possess much motion.
All animals have a muscular structure entering into their composition, with some variety in its appearance. Muscles are generally fibrous to the eye, and in Man and Quadrupeds are of a red color; in some animals, however, these circumstances are not at all obvious. Thus in many fishes, the muscles are white, and put on a flaky appearance; whilst in the fresh water Polypus, which possesses a great degree of contractile power, no fibres can be seen. So that it is not necessary that these properties should be obvious in the muscles of all animals. Thus no person has ever seen the fibres in the muscles of a Flea, yet no animal can exert greater muscular power. In the same way, many parts of the body possess a contractile power, which have no apparent fibrous structure; the best example of this, is the skin of the scrotum. The redness of a muscle, in fact, depends in a great measure on the degree of exertion it undergoes; thus when a limb becomes motionless from palsy, the muscles uniformly become pale.—The function of a muscle consists in its contracting or shortening itself, in consequence of the application of certain stimuli or excitements; the effect of this contraction is, that the different bones to which the muscles are attached are moved in various directions. Thus (to give an example) a muscle affixed to two contiguous bones, by shortening itself, brings those points to which it is affixed nearer to each other; and, from this mechanism, arise all the motions of the body. The greatest part of the muscles which put the limbs in motion by their contractions, are said to act under the excitement of volition, or, in other words, are under the control and influence of the will, and are therefore called voluntary muscles. There are many muscles, however, which are not excited by volition, and are therefore called involuntary. As these are directed by influences, and perform the actions on which life immediately depends, they, for obvious reasons, are not only put beyond the powers of the will, but are enabled to carry on their contractions and motions without interruption or fatigue, entirely independent of its direction or our consciousness. In this manner the heart performs the circulation of the blood, and the stomach and intestines give the requisite motion to the food.—There are many other excitements which produce contraction in muscles, such as the passions and emotions of the mind, and various mechanical and chemical stimuli. Some of them occasionally excite the voluntary muscles of the body to a degree of action, over which volition has no control. Thus a person in an ordinary state of mind, can walk more or less quietly as suits his convenience; but it occasionally happens, we shall say in the field of battle, that the passion of fear is excited; this excitement frequently disregards the power of the will, and strongly excites the muscles employed in running away.[197]