450 VOLUNTARY AND INVOLUNTARY ACTIONS OF MUSCLES. more or less under the control of the will, they may all at times have an involuntary action. The heart and the muscular coat of the alimentary canal, with the muscles concerned in swallowing and in one or two other actions of a similar character, are the only muscles which the will cannot either set in action, or control when in action. There are several muscles whose usual movements are of a reflex and therefore involuntary character, and are yet capable of being, to a certain extent, controlled and governed by the will. Such are the movements of respiration; which will continue to take place after the brain has been removed, and which go on regularly during the profoundest sleep and the most complete withdrawal of the attention from them. In the Invertebrated animals these motions are probably not influenced by the will; but in the air-breathing Vertebrata they are placed in a certain degree under the dominion of the will, in order that they may be made to contribute to the production of the vocal actions of speaking, singing, &c., which are restricted to these classes. We can hold the breath for a certain time by a voluntary effort, or we can expel or draw it in more quickly than usual; but no voluntary effort can cause the breath to be held for more than a few moments; for the uneasiness which is then felt, and which is continually increasing, causes an involuntary action of the muscles, by which action it is relieved. 590. But again, there are other muscles, whose ordinary actions are voluntary; but which are occasionally made to act independently of the will, or even against its direction. Such are those which are excited by the emotions, as in laughing, crying, sobbing, &c. We may have the strongest desire to check these actions, owing to the unfitness of the time and place for their manifestation; and yet we may be unable to do so. And lastly, muscles whose action is usually voluntary may be occasionally called into powerful contraction, which the will cannot in the least degree control or prevent; this is the case in cramps, convulsions, &c., of various kinds.-All these facts are readily accounted-for by the knowledge we now possess, as to the functions of the different parts of the nervous centres from which the muscles receive their stimulus to action (Chap. x.). 591. The vigorous action of the muscular structure is de CONDITIONS OF MUSCULAR ACTIVITY. 451 pendent upon several conditions. In the first place, it requires an active nutrition of the muscles themselves. Firm, plump, and high-coloured muscles act with greater force than those which are pale and flabby, even though the size of the latter may be greater. Again, in all those animals whose activity is greatest, a constant supply of oxygen is requisite for muscular vigour. This, like the nutritive material, is conveyed, in Birds and Mammals, by the blood (§ 235); in Insects, on the other hand, it actually enters the muscular tissue in the state of atmospheric air (§ 321). In Reptiles, again, the blood goes to the tissues very imperfectly oxygenated; and their movements are comparatively slow and feeble. But it is a remarkable circumstance, that in the dead bodies of the latter, or in parts separated from the living body, the property of contractility does not depart nearly so soon as it does in similar parts of warm-blooded animals. By experiments on Mammals it has been found that the muscles of the trunk cannot be caused to contract by galvanism for more than two or three hours after death, though the auricles of the heart retain their contractility for some hours later. The muscles of Birds (whose respiration is more active, and whose temperature is higher) lose their contractility yet sooner; but those of Reptiles sometimes retain the power of contracting for several days. When venous or imperfectly-aerated blood is made to circulate through the vessels of warm-blooded animals, it acts like a poison upon them, diminishing or even destroying their contractility.1 592. Further, the energy of muscular contraction depends in great degree upon the power of the stimulus which is transmitted to it through the nervous system. We often have the opportunity of observing this, in the case of persons who are under the excitement of violent passion or of insanity; a delicate female becoming a match for three or four strong men, and even breaking cords and bands that would hold the most powerful man in his ordinary state. The strength in such circumstances seems almost preternatural; but it is not 1 Other substances do this with even greater rapidity; thus a strong solution of nitrate of potass (nitre) injected into the blood-vessels, and conveyed by them to the heart, causes the immediate cessation of its action, the poison finding its way, through the vessels of the organ itself, into the capillaries of its muscular structure. 452 FATIGUE.-ENERGY AND RAPIDITY OF MOVEMENT. greater than that which we see manifested in convulsive actions, where the movements depend only upon the reflex activity of the spinal cord. Thus a slender girl affected with a spasmodic affection of the muscles of the spine, which threw the back into an arch of which the head and the heels were the two resting-points, has been known to raise a weight of 900lbs. laid on the abdomen with the absurd intention of straightening the body. 593. The sense of fatigue, which comes-on after prolonged muscular exertion, is really dependent upon a change in the brain, though usually referred by us to the muscles that have been exercised. For it is felt after voluntary motions only; and the very same muscles may be kept in reflex action for a much longer time, without any fatigue being experienced. Thus, we never feel tired of breathing; and yet a forced voluntary action of the muscles of respiration soon causes fatigue. The voluntary use of the muscles of our limbs, in walking or running, soon occasions weariness; but similar muscles are used in Birds and Insects, for very prolonged flights, without apparent fatigue; and as we find that the actions of flight may be performed, after the brain, or the ganglia that correspond to it in Insects, have been removed (§§ 444, 465), we may regard them as of a reflex character; and the absence of fatigue is thus accounted-for. 594. The energy of muscular contraction appears to be greater in Insects, in proportion to their size, than it is in any other animals. Thus a Flea has been known to leap sixty times its own length, and to move as many times its own weight. The short-limbed Beetles that inhabit the ground have an enormous power, which is manifested both in their movement of heavy weights, and in the resistance they overcome with their jaws. Thus the Dung- or "shard-borne " Beetle can support uninjured, and even elevate, a weight equal to at least 500 times that of its body. And the Stag-Beetle has been known to gnaw a hole of an inch diameter, in the side of an iron canister in which it had been confined. The rapidity of the movements of Insects is also most extraordinarily great, and is especially seen in the vibrations of their wings. It would be impossible to form an estimate of the time occupied by these, were it not for the musical tones they produce; and it may be calculated from these that the wings of APPLICATIONS OF MUSCULAR POWER. 453 many Insects strike the air several hundred times,—and those of some of the smaller Insects many thousand times,in a second of time. Applications of Muscular Power:-Bones and Joints. - 595. Muscular contraction performs an important part in nearly every one of the functions of which we have already treated. Thus the reception of the food, and its propulsion along the alimentary canal, forming part of the function of Digestion, are accomplished through its means. The Circulation of the blood, again, depends mainly on the agency of a contractile organ, the heart. The Respiration cannot be kept up, in the higher animals at least, without the aid of certain movements which are accomplished by the muscles. With the processes of Nutrition and Secretion it is not so closely connected; but the latter is dependent upon it so far as this, that its products are carried out of the body by the aid of muscular contraction. And even in Sensation, the peculiar endowment of muscular tissue comes into use; by giving to the organs of sense those movements which enable them to take a wider range, and to apply themselves most perfectly to the objects before them. But we have now to study its applications in those general and partial movements of the body, on which depend the locomotion (or change of place) of animals, their attitudes, and a number of other important actions, entirely of a mechanical nature. 596. The organs by which these are effected, may be conveniently divided into the active and passive. The active are those which have peculiar vital powers within themselves, and which exert these in giving motion to other parts. To this class, therefore, we refer the Muscles, whose peculiar endowments have been just considered. The passive organs, on the other hand, are those which perform no action of themselves, which have no power but that of yielding a simply mechanical support, and which consequently perform no movement but such as they are made to do by the muscles. Of this kind are the hard parts which form the skeleton or solid framework of the body, whether this be internal or external. 597. In the lower tribes of animals, the muscles are all inserted in the soft and flexible membrane which covers the body; and it is by acting upon this, that they can change the 454 APPLICATIONS OF MUSCULAR POWER :-SKELETON. form of the body in such a manner as to cause it to move, either altogether or in part. This is the case, for example, in the Leech, Earth-worm, and other Annelids; which are furnished with two sets of muscular fibres, one running along the body, and the other passing round it in rings. By the contraction of the former, the two ends are drawn together, so that the body is shortened; whilst by that of the latter, its diameter is lessened, so that it is necessarily lengthened. By these two movements, which take place alternately, the progression of the animal is accomplished; and by varying the contractions of one part or another, almost any form and direction can be given to the soft and flexible body. 598. But in the higher animals we find the apparatus of movement to consist, not only of muscles, but also of a framework of solid pieces, which serves to augment the precision, the force, and the extent of the movements; whilst, at the same time, it determines the general form of the body, and protects the viscera against injury from without. This solid framework, or skeleton, to which the muscles are attached, may be, as we have seen, either internal or external. In the Vertebrated classes, the hard skeleton is internal; in the Articulated series it is external; in the Mollusks it is external, but does not afford fixed attachments to muscles, except to such as draw together its valves, or connect it with the soft body of the animal; and in the Radiata its position is variable, being sometimes external as in the Echinodermata, and sometimes internal as in the stony Corals. 599. The skeleton of Vertebrata differs from that of all the Invertebrated classes in the much higher character of its organization, which enables it to grow with the growth of the body generally, not merely in virtue of the additions it receives, but by the successive removal of its previously formed parts as occasion may require; so that the skeleton of the adult has been entirely substituted for that of the child, probably no part of the latter being contained in the former. The skeletons of the Invertebrata, where they are not formed of horny matter alone, are consolidated by carbonate of lime, which in some instances (as the shells of many Mollusks, and the Stony Corals) bears so large a proportion to the animal basis, that the latter can scarcely be detected. The growth of these skeletons takes place entirely by additions to |