286 SCIENCE OF AGRICULTURE. Part II. 



texture, and possess great tenacity. The threads of which they consist, are attached 

 on the one extremity to the surface of a bone, or other hard part ; and on the other, they 

 are variously interspersed among the fibres or bundles of the muscle. They are consi- 

 dered as destitute of sensibility and irritability, and form a passive link betvi^een the 

 muscle and the bone, or other point of support. 



1856. Muscles are the most active members of the animal frame. They alone possess the power of 

 irritability and execute all the motions of the body. The causes which excite them to action, maybe 

 reduced to' two kinds. In the first the will, through the medium of the nerves, excites the irritability of 

 the fibres and in the second, the action is produced by the application of external objects, either directly 

 or by the medium of the nerves. The changes which take place in the tenacity of muscles after death, 

 are very remarkable. The same force which they could resist with ease, in a living state, is sufficient to 

 tear them to pieces after the vital principle has departed. 



1857. The functions of the muscles are either those of rest or motion. Many animals 

 protect themselves against the disturbing movements of the air and water, by placing 

 their bodies in a prone position. To give still greater efficacy to this protecting attitude, 

 they retire to valleys, woods or dens, on the earth, or to the deepest places in the waters ; 

 and are thus able, by the weight of their own bodies, and the advantage of their 

 position, to outlive the elemental war. But there are other animals, which, while they 

 are equally cautious to make choice of proper situations for their safety, employ in 

 addition, peculiar organs with which they are provided, to connect themselves more 

 securely with the basis on which they rest. 



1858. Grasping. The most simple of these expedients, grasping, is displayed by bats, birds and insects, 

 in the employment of their toes, with their claws, in seizing the objects of their support. In birds, the 

 assumption and continuance of this attitude is accomplished by a mechanical process ; so that there is no 

 expenditure of muscular energy. In every case of this kind, the claws are so admirably adapted to the 

 station of the animal, that the detention of the body in the same spot during this state of rest, is accom- 

 panied with little exertion. 



1859. Suction. The third method of fixing themselves employed by animals, is suction. The sucker 

 varies greatly in its form, and even structure. In the limpet, and other gasteropodous mollusca, its surface 

 is smooth and uniform ; and the adhesion appears to depend on its close application to every part of the 

 opposing surface. In other animals, as the leech and the sea-urchin, the sucker is formed at the extremity 

 of a tube ; the muscular motions of which may serve to pump out any air which may remain, after the 

 organ has been applied to the surface of the body. 



1860. Cementation The fourth method, termed cementation, employed by animals to preserve them- 

 selves stationary, consists in a part of their own bodies being cemented to the substance on which they 

 rest This takes place in the common mussel, by means of strong cartilaginous filaments, termed the 

 byssus, united in the body to a secreting gland, furnished with powerful muscles, and, at the other extre- 

 mity, glued to the rock or other body to which it connects itself. In other cases, as in the oyster, the shell 

 itself is cemented to the rock. 



1861. The muscular motions of animals are standing, walking, leaping, flying, and 

 swimming. 



1S62. In standing it lis necessary that the parts of the body be so disposed, as that the centre of gravity 

 of the whole body fall within the space which they occupy, and that the muscles have suflScient power to 

 counteract those movements which might displace the body from that position. It is obvious that the 

 more numerous the limbs, and the more equally they are distributed on the inferior side of the body, the 

 more securely will the centre of gravity be retained within the space which these feet include. 



1863. Walking is defined by Cuvier, to be a motion on a fixed surface, in which the centre of gravity is 

 alternately moved by one part of the extremities, and sustained by the other, the body never being at 

 any time completely suspended over the ground. It is produced by the alternate flexion and extension 

 of the limbs, aided" by the motions of the trunk, advancing the position of the centre of gravity in the 

 intended direction. 



1864. In animals with many feet, as the myriajioda, walking is performed by so uniform 

 a motion, that the body may be said to glide along the surface. 



1865. In animals with four feet, " each step is executed by two legs only ; one belonging to the fore 

 pair, and the other to the hind pair ; but sometimes they are those of the same side, and sometimes 

 those of the opposite side." {Cuvier's Comparative Anatomy, Lect. vii. a, 1.) The latter is that kind of mo- 

 tion in horses, which grooms term apace. The right fore-leg is advanced so as to sustain the body, which 

 is thrownupon it by the left hind-foot, and at the same time, the latter bends in order to its being moved 

 forward. While they are off the ground, the right-hind food begins to extend itself, and the moment 

 they touch the ground, the left fore-foot moves forward to support the impulse of the right foot, which 

 likewise moves forward. The body is thus supported alternately by two legs placed in a diagonal manner. 

 When the right fore-foot moves, in order to sustain the body, pushed forward by the right hind-foot, the 

 motion is then called an amble. The body, being alternately supported by two legs on the same side, is 

 obliged to balance itself to the right and left, in order to avoid falling ; and it is this balancing movement 

 which renders the gait so soft and agreeable to women and persons in a weak state of body. {Cuvier's 

 Comp. Anat. Lect. vii.) 



1866. The serpentine motion consists in bringing up the tail towards the head by bending the body into one 

 or more curves, then resting upon the tail, and extending the body, thus moving forward, at each step, 

 nearly the whole length of the body, or one or more of the curves into which it was formed. Among 

 the mollusca, and many of the annulose animals, the same kind of motion is performed by alternate 

 contractions and expansions, laterally and longitudinally of the whole bo<ly, or of those parts which 

 are appropriated to progressive motion. 



1867. A?m)de of moving analogous to walking, is performed by animals who have suckers, and is 

 exemplified in the leech, which at every step advances nearly the whole length of its body. 



1868. In the action of leaping, the whole body rises from the ground ; and for a 

 short period is suspended in the air. It is produced by the sudden extension of the 

 limbs, after they have undergone an unusual degree of flexion. The extent of the leap 

 depends on the form and size of the body, the length and strength of the limbs. The 

 myriapoda are not observed to leap. Many of the spiders and insects leap with ease both. 



