are developed, this intimate relation between the muscles

and the vertebræ diminish

es. The muscles are unequally distributed and are concentrated

about the

limbs, where the greatest amount of muscular force is required. For this reason, the largest masses of flesh in the higher vertebrates are found about the shoulders and hips; while in fishes they are concentrated about the base of the

Fig. 30.

tail, which is the part principally employed in locomotion.

SECTION II.

OF LOCOMOTION.

163. One of the most curious and important applications of this apparatus of bones and muscles is for LocoмoTION. By this is understood the movement which an animal makes in passing from place to place, in the pursuit of pleasure, sustenance, or safety, in distinction from those motions which are performed equally well while stationary, such as the acts of respiration, mastication, &c.

164. The means which nature has brought into action to effect locomotion under all the various circumstances in which animals are placed, are very diversified; and the study of their adaptation to the necessities of animals is highly interesting in a mechanical, as well as in a zoological point of view. Two general plans may be noticed, under which these varieties may be arranged. Either the whole body is

equally concerned in effecting locomotion, or only some of its parts are employed for the purpose.

165. The jelly-fishes (Medusa) swim by contracting their umbrella-shaped bodies upon the water below, and its resistance urges them forwards. Other animals are provided with a sac or siphon, which they may fill with water and suddenly force out, producing a jet, which is resisted by the surrounding water, and the animal is thus propelled. The Bîche-le-mar, (Holothuria,) the cuttle-fishes, the Salpæ, &c., move in this way.

Fig. 31.

166. Others contract small portions of the body in succession, which being thereby rendered firmer, serve as points of resistance, against which the animal may strive, in urging the body onwards. The earth-worm, whose body is composed of a series of rings united by muscles, and shutting more or less into each other, has only to close up the rings at one or more points, to form a sort of fulcrum, against which the rest of the body exerts itself in extending forwards.

167. Some have, at the extremities of the body, a cup or some other organ for maintaining a firm hold, each extremity acting in turn as a fixed point. Thus the Leech has a cup or sucker at its tail, by which it fixes itself; the body is then

elongated by the contraction of the muscular fibres which encircle the animal; the mouth is next fixed by a similar sucker and by the contraction of muscles running lengthwise the body is shortened, and the tail, losing its hold, is brought forwards to repeat the same process. Most of the bivalve mollusks, such as the clams,

Fig. 32.

A fleshy organ,

move from place to place, in a similar way. called the foot, is thrust forward, and its extremity fixed in the mud, or to some firm object, when it contracts, and thus draws along the body and the shell enclosing it. Snails, and many similar animals, have the fleshy under surface of their body composed of an infinitude of very short muscles, which, by successive contractions, so minute, indeed, as scarcely to be detected, enable them to glide along smoothly and silently, without any apparent muscular effort.

168. In the majority of animals, however, locomotion is effected by means of organs specially designed for the purpose. The most simple are the minute, hair-like cilia, which fringe the body of most of the microscopic infusory animalcules, and which, by their incessant vibrations, cause rapid movements. The sea-urchins and star-fishes have little thread-like tubes issuing from every side of the body, furnished with a sucker at the end. By attaching these to some fixed object, they are enabled to draw or roll themselves along; but their progress is always slow. Insects are distinguished for the number and great perfection of their organs of motion. They have at least three pairs of legs, and usually wings also. But those that have numerous feet, like the centipedes, are not distinguished for agility. The Crustacea generally have at least five pairs of legs, which are used for both swimming and crawling. The Worms are much less active; some of them have only short bristles at their sides.

Some of the marine

Fig. 33.

species use their fringe-like gills for paddles. (Fig. 33.) 169. Among the Vertebrata, we find the greatest diversity in the organs of locomotion and the modes of their application, as well as the greatest perfection, in whatever element

they may be employed. The sailing of the eagle, the bounding of the antelope, the swimming of the shark, are not equalled by any movements of insects. This superiority is due to the internal skeleton, which, while it admits a great display of force, gives to the motions, at the same time, a great degree of precision.

1. Plan of the Organs of Locomotion.

170. The organs of progression in vertebrated animals never exeeed four in number, and to them the term limbs is more particularly applied. The study of these organs, as characteristic of the different groups of vertebrate animals, is most interesting, especially when prosecuted with a view to trace them all back to one fundamental plan, and to observe the modifications, oftentimes very slight, by which a very simple organ is adapted to every variety of movement. No part of the animal structure more fully illustrates the unity of design, or the skill of the Intellect which has so adapted a single organ to such multiplied ends. On this account, we shall illustrate this subject somewhat in detail.

171. It is easy to see that the wing which is to sustain the bird in the air must be different from the leg of the stag, which is to serve for running, or the fins of the fish that swims. But, notwithstanding their dissimilarity, the wing of the bird, the leg of the stag, and the shoulder fin of the fish, may still be traced to the same plan of structure; and if we examine their skeletons, we find the same fundamental parts. In order to show this, it is necessary to give a short description of the composition of the arm or anterior extremity.

172. The anterior member, in the vertebrates, is invariably composed of the following bones: 1. The shoulderblade, or scapula, (a,) a broad and flat bone, applied upon .ne bones o he trunk · 2. The arm, (b,) formed of a single

a

long cylindrical bone, the humerus; 3. The fore-arm composed of two long bones, the radius, (c,) and ulna, (d,) which are often fused into one; 4. The hand, which is composed of a series of bones, more or less numerous in different classes, and which is divided into three parts, namely, the carpus, or wrist, (e,) the metacarpus, or palm, (ƒ,) and the phalanges, or fingers, (g.) The clavicle or collar-bone, (0,) when it exists, belongs also to the anterior member. It is a bone of a cylindrical form, fixed as a brace between the breast-bone and shoulder-blade. Its use is to keep the cshoulders separated; to this end, we find it fully developed in all animals which raise the limbs from the sides, as the birds and the bats. On the other hand, it is rudimentary, or entirely wanting in animals which move them backwards and forwards only, as with most quadrupeds.

Fig. 34.

-b

cd

Fig. 35.

173. The following outlines, in which corresponding bones are indicated by the same letters, will give an idea of the modifications which these bones present in different classes. In the arm of man, (Fig. 34,) the shoulder-blade is flat and triangular; the bone of the arm is cylindrical, and enlarged at its extremities; the bones of the fore-arm are somewhat shorter than the humerus, but more slender; the hand is composed of the following pieces, namely, eight small bones of the carpus, arranged in two rows, five metacarpal bones, which are elongated, and succeed those of the wrist; five fingers of unequal length, one of which, the thumb, is opposed to the four others.