Haying discovered how the bones are joined by arti culating surfaces, and preserved in their situation by connecting substances, we must now inquire by what means they are put in motion. On the slightest exa mination, we perceive, that all the motions in the body are produced by means of bundles of flesh, which have received the name of muscles. These bundles, or fas ciculi, frequently consist of two parts ; the one flesh, the other a white glistening substance, not unlike ligament, which, in common language, is called sinew, but is bet ter known in anatomy by the name of tendon. Both the fleshy and tendinous parts arc composed of fibres, which again are composed of others still smaller. The con stituent fibres have never been enumerated, nor indeed all the fascieuli tthich they compo-,e. ['he number of those fasciculi which Lim e been described and minted, reckoning classes sometimes as pairs, amounts to nearly four hundred. Toe branch ()I which treats of these organs, is styled myolo,:;.y ; a knowledge of which is extremely useful in the treatment of dislocations and fractures; and to those who study gesture and attitude. and the various movements of different animals, in swimming, walking, leaping, or flying—perhaps di( most curious and interesting part of the whole science. The motions of these organs are performed by alternate contractions and relaxations of the fleshy part; but these motions seem to depend on sonic other cause, which it therefore becomes necessary to investigate.
On minute examination, white filaments, or ropes, at observed entering, not only the several muscles, but all the different organs of the body ; and, when traced to their origin, they arc found to terminate in the brain, or in its production, the spinal marrow. These filaments, which are called nerves, besides being necessary in all the different functions of the system, have been found, by observation and experiment, to be organs of sense. motion, and feeling. Many of them are subject to the will ; so that the animal can regulate a variety of its motions at pleasure. They derive their energy from the vital power; but whether immediately, or through the medium of sonic other substance, anatomists have not been able to discover. They separate into numerous filaments, which, in many instances, cannot be traced. Some of them swell into small knots, called ganglia; and some of them form, by means of other filaments, a sort of network, which is known by the name of plexus. Forty trunks issue corn the head and spine; but these trunks, with their ganglia and plexus, their branches and parts which have been distinguished by names, amount to more than a hundred.
Having thus seen how the bones are joined, by what substances they arc connected, by what organs they are put in motion, and what is the cause of that motion, we must next investigate the means of their growth. Growth necessarily supposes a regular and constant supply of nourishment. Whence, then, is the nourish
ment derived ? and by what means is it conveyed to the several organs ? On examining the animal structure, we find, that the only vehicle of nourishment must he the blood, which flows through a thousand channels into all the different parts of the system. The blood, when al lowed to rest, spontaneously separates into two parts. the one fluid and the other solid; by a slight pressure, the solid is again divided into other two; but chemical analysis proves, that it consists of many different ingre dients. Every person is acquainted with the singular colour of this fluid. Lower observed, that it received this colour in the lungs, or the organs of breathing. Mayow demonstrated, that the colour was occasioned by the air, which we regularly inspire; and experience has taught, that without this regular supply of air, the blood could neither support life, nor convey nourishment to the different parts of the system. It is natural to suppose, that the heart, which is the great fountain of the blood, will send some to the lungs, in order to re ceive this pabulum, which Mayow likewise proved to be the cause of animal heat. Man, accordingly, and many other animals, are furnished with a double heart, or at least two cavities, which are named ventricles; one for sending the blood to the lungs to receive air, and the other for distributing it afterwards for the sake of nourishment through the system, and even to the heart, and the lungs themselves.
To explain the cause of the constant and impetuous flow ol the blood, it is necessary to ihform our readers, that the heart is a muscle, which, through the influence of its nervous energy, is made to contract and relax al ternately more than sixty times in a minute. When it relaxes, its two cavities, or ventricles are enlarged, and the blood flows in ; when it contracts, the ventricles arc diminished, and the blood is propelled into two large pipes; the one leading directly to the lungs, the other communicating with tne rest ol the system. These pipes, or vessels, are named arteries; and as they likewise have a muscular power, and are somewhat elastic, by similar contractions, which are the cause of pulsation, they impel the blood towards their extremities, while certain valves, placed at their origin, prevent it front returning in the same direction.
For the more minute and equal distribution of this fluid, the two arteries which proceed from the heart spread into a great variety of branches. These branches frequently communicate, apparently with this intention of nature, that when any of them are obstructed, the parts may be duly supplied by others with which they inosculate. These various branches are numerous be yond computation: many of them, however, are de scribed by anatomists ; and those branches, or parts of branches, which have received names, reckoning classes sometimes as pairs, atrount to rather more than a hun dred.