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CHAPTER XII.

MOTION.

Organs of Motion consist of the Passive and the Active-Bones-Their Textures and various Forms-Their Chemical Constitution-Adaptation to the purposes they subserve-Variety of Connexion-Gristle-Its pecular Organization-lus great Elasticity-Forms the Principal Springs of the Animal Machine-Subservient to Mechanical Movements-Elastic Cushions-Ligament-Its Structure—A very Common Tissue in Animal Bodies-Presents great varieties, and subservient to many purposes-Elastic Ligamentous Tissue a substitute for Muscular Power -Continues its Action in the Dead Body-Tendon-Varieties in the Form of— Bears concentrated Muscular Force-Transfers Muscular Power-Muscular Vibre-Characteristic Property Vital-Organization-Chemical Constitution— Muscular Action-Changes in the State of Action and Repose-Exciting causes of Muscular Action-The Will-Instinct and Sympathy-Involuntary Muscular Motion-Influence of Disease-Mechanical and Chemical Excitants-Remarkable Effects of Galvanism on the Body of an Executed Criminal-Various Modifications of the Actions of Muscles in the Preservation of the Attitudes, and in Performance of Motion-Force of Muscles-How modified-Muscular Vigour in Different Races-Duration of Action-Velocity of Motion-In ManIn Animals-Mutual adaptation of Structure and Function in Animal Bodies.

In the last four chapters we have had under consideration the instruments by which we become acquainted with the external world around us. We have now to inquire into the mechanical means whereby we are enabled to react on surrounding objects, to change our positions, to fix our attitudes, and to exercise several of those functions to which our attention has already been directed. For these purposes the higher classes of animals are furnished with several distinct organs, of various textures and adaptations, differing in their forms, number, proportion and structures, according to the rank and circumstances of the animal.

The organs subservient to the form, motions, stability,

and postures of the body, consist of two classes, the passive and the active. To the former belong the bones, cartilages, ligaments, and tendons; the latter comprises the muscular fibre. A few general observations respecting each of these tissues will be necessary.

Bone. The bones constitute the hardest and most solid parts of the whole system, and are the principal parts that give it form, stability, and posture, forming, as it were, the frame-work of the animal machine. In man and the higher order of animals, the skeleton, generally speaking, is internal; while in the lower tribes, as insects and crustacea, such as the lobster, crab and others, it is placed externally. Including the teeth, the number in the human adult amounts to about 245. They are usually arranged in three classes, the long or cylindrical, such as the bones of the arms and legs, the broad flat bones, as the shoulder-blade, and the round or angular bones, as those of the wrist and ankle. When the broken surface of a bone is examined under a good microscope, it presents an uniform surface, abundantly supplied with minute bloodvessels, without any appearance of fibres or plates. The different densities which bones present, arise from differences in the mechanical arrangement of the particles composing them. In the thigh-bone, for example, the extremities bulge out so as to afford a more extensive surface for the insertion of muscles and ligaments, and to increase their levers by throwing them further from the centre of motion. In the shaft, where strength is chiefly required, the surface is compact, either surrounding a spongy, cellular, or cancellated structure, or forming a hollow tube filled with marrow, thus in the most perfect manner combining the greatest degree of strength with the least degree of weight and expense of material. It can be mathematically demonstrated that the resistance of a cylindrical body, such as a pillar or mast, to a force applied transversely, is increased in proportion to its diameter. The same quantity of matter, therefore, placed

to taste admits of doubt, as there are no means of determining the fact. Other parts of the mouth, however, may act in this way, as the soft and irritable body on the palate of the carp.

That many of the invertebrated animals are endowed with taste, we have every reason to believe; such as bees, wasps, flies, and leeches. The organs subservient to this purpose, however, have not as yet been satisfactorily ascertained. The condition in which substances most readily act upon the sense (if indeed it be not absolutely essential) is the state of solution. Of the three conditions of matter, then, touch judges most correctly of solids, taste appreciates fluids, and smell takes cognisance of gaseous bodies. In order to excite smell, it is necessary that the odorous particles be inhaled with the breath, and drawn with it through the nostrils, where they come in contact with the immediate seat of the sense. With respect to savours, it is requisite that they be dissolved and brought in contact with the papillæ, in which twigs of the gustatory nerves are incorporated, on which the impressions are made, subquently to be transmitted to the seat of perception.

The intimate connexion subsisting between smell and taste is sufficiently demonstrated in the closure of the nostrils, when the latter becomes much blunted, and almost abolished; wherefore it is nearly impossible to distinguish between the tastes of different substances, as between sweet and bitter, sour and alkaline, and so forth, a circumstance that is occasionally taken advantage of when nauseous medicines are to be administered, by shutting the mouth when the draught is swallowed.

It has been already observed, that neither smell nor taste rank high as channels of intellectual information, although by cultivation their powers are much extended, and their accuracy vastly improved. Dr Kitchener asserts that some epicures are actually able to tell from what precise reach of the Thames a salmon had been caught, when presented at table. But such refinement in

either of these senses may truly be held as by no means desirable, since they are liable to so many causes that excite in them offence and annoyance, rather than gratification and satisfaction. Yet in many of the lower animals they are no doubt of the most essential importance in enabling them to select that kind of food that is proper for them, and in rejecting such as might prove prejudicial.

to taste admits of doubt, as there are no means of determining the fact. Other parts of the mouth, however, may act in this way, as the soft and irritable body on the palate of the carp.

That many of the invertebrated animals are endowed with taste, we have every reason to believe; such as bees, wasps, flies, and leeches. The organs subservient to this purpose, however, have not as yet been satisfactorily ascertained. The condition in which substances most readily act upon the sense (if indeed it be not absolutely essential) is the state of solution. Of the three conditions of matter, then, touch judges most correctly of solids, taste appreciates fluids, and smell takes cognisance of gaseous bodies. In order to excite smell, it is necessary that the odorous particles be inhaled with the breath, and drawn with it through the nostrils, where they come in contact with the immediate seat of the sense. With respect to savours, it is requisite that they be dissolved and brought in contact with the papillæ, in which twigs of the gustatory nerves are incorporated, on which the impressions are made, subquently to be transmitted to the seat of perception.

The intimate connexion subsisting between smell and taste is sufficiently demonstrated in the closure of the nostrils, when the latter becomes much blunted, and almost abolished; wherefore it is nearly impossible to distinguish between the tastes of different substances, as between sweet and bitter, sour and alkaline, and so forth, a circumstance that is occasionally taken advantage of when nauseous medicines are to be administered, by shutting the mouth when the draught is swallowed.

It has been already observed, that neither smell nor taste rank high as channels of intellectual information, although by cultivation their powers are much extended, and their accuracy vastly improved. Dr Kitchener asserts that some epicures are actually able to tell from what precise reach of the Thames a salmon had been caught, when presented at table. But such refinement in

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