or their laws of action.

He takes these as he finds them.

His skill and ingenuity are employed in turning them, such as they are, to his account, by giving to the parts of his machine. a form and relation, in which these unalterable properties may operate to the production of the effects intended.

The muscular system would afford us numerous examples of what may be called mechanical structure: i. e. of such contrivances employed to attain certain objects, as a human artist would adopt on similar occasions. One of the muscles of the eye-ball presents us with a very perfect pulley; by means of which the globe of the eye is moved in a direction exactly contrary to the original application of the force. This muscle, which is called the trochlearis, arises from the very back part of the orbit; it has a long and slender tendon running through a pulley in the inner part of the front margin of the orbit, and then going back to be fixed in the hind portion of the eye-ball. Thus it draws the globe obliquely upwards and forwards, although the line of the contraction of the muscle is directly backward.

In the toes and fingers, the long tendon, which bends the first joint, passes through the short tendon, which bends the second joint.

The foot is placed at a considerable angle with the leg. It is manifest, therefore, that flexible strings, passing along the interior of the angle, if left to themselves, would, when stretched, start from it. The obvious preventive is to tie them down, and this is done in fact. Across the instep, or rather just above it, the anatomist finds a strong ligament, under which the tendons pass to the foot. The effect of the ligament, as a bandage, can be made evident to the senses; for if it be cut, the tendons start up. The simplicity, yet the clearness of this contrivance, its exact resemblance to established resources of art, place it among the most indubitable manifestations of design, with which we are acquainted.

Of Muscular Motion. Muscular motions are of three kinds, viz. voluntary, involuntary, and mixed. The voluntary

motions of the muscles, are such as proceed from an immediate exertion of the active powers of the will: thus the mind directs the arm to be raised, the knee to be bent, the tongue to speak, &c. The involuntary motions of muscles are those which are performed by organs seemingly of their own accord, and without any attention of the mind, or consciousness of its active power; as the contraction and dilatation of the heart, arteries, veins, stomach, &c. The mixed motions are those which are in fact under the control of the will, but which usually act without our being conscious that they do so, as in the case of the muscles of respiration.

As motion is produced by the muscle contracting both its ends towards the centre, when one end is fixed, the other must of course be drawn to the centre of motion, and with the bone or any other part to which it is affixed; and thus by the cooperation of several muscles, not only a limb, but even the whole body is put into action. This is the case with all the muscles of voluntary motion; their fibres contract on the application of the nervous influence, and the whole muscle shortens itself, and on this same principle the other muscles perform involuntary motion. The heart contracts from the stimulating properties of the blood; the arteries do the same; also the absorbent vessels by a similar action of their contents; and likewise all those organs and parts which have the power of acting, independently of the mind. Hence the motions of animals have been defined to be the contraction of the muscular fibre, from the presence of some stimulating influence. But it has not yet been discovered whence the muscular fibre derives this contractile power.

OF THE BRAIN AND NERVES. The brain is a soft and whitish substance, situated in the cavity of the skull, and corresponding in form to that cavity. Its parts are supported by a firm membrane, called the dura mater; and its substance is more immediately invested by a delicate membrane, called the pia mater. The structure of the brain is remarkably con

stant and uniform; very seldom deviating from the accustomed standard; which seems to prove, that the right performance of the functions of this organ requires a great exactness in the structure of the individual parts. The whole brain is divided into two parts: that which is in the upper or fore part of the skull, is called the Cerebrum; and that which lies at the back part just under the Cerebrum, is denominated the Cerebellum. The dura mater acts as a lining to the inside of the skull, and it has three parts or processes serving as partitions to certain portions of the brain to keep it steady. The pia mater invests the brain even between its lobes and folds. It serves not only to contain the brain, but to support its blood-vessels, which are in this part in great numbers; that the blood may not enter the brain too impetuously, the veins also unite upon it.

There is likewise a medullary production from the under part of the two divisions of the brain, which is called the Medulla Oblongata. The production of this through the great opening of the skull, and down the channel of the spine, is the Medulla Spinalis, or Spinal Marrow.

It may be observed, as shewing the characteristics of the three substances just enumerated, that wounds in the Cerebrum, though very dangerous, are not mortal; but in the Cerebellum and Medulla Oblongata, they cause sudden death; and in the spinal marrow, they occasion loss of sense, in all the parts which receive nerves from below the wound.

The nerves are soft, white, fibrous cords, that arise from the brain and spinal marrow: they come out in pairs, and are distributed through the whole body. There are forty pairs of nerves; of these, nine pair arise from the base of the brain within the skull; a tenth from the brain, as it passes through the great hole of the skull into the spine; and the other thirty pair proceed from the spinal marrow. Those arising from the brain pass through holes in the base of the skull, and are distributed chiefly to the organs situated in the head, and to those contained in the chest and belly: while the nerves, which arise from the spinal marrow go, partly among the internal organs

of the trunk, to be distributed to the exterior parts of the body' and to the extremities, or limbs.

Respecting the structure of the brain and nerves-the nature of their powers, and in what way the operations of the mind are connected with the matter of the brain, nothing yet has been ascertained. The following facts are, however, generally admitted as true: 1. The brain and nerves are sensible, constituting the organs of feeling and sensation in the animal machine. 2. All the other parts of the body derive their power of feeling and sensation from the brain, the spinal marrow, and the nerves; being in themselves wholly insensible, and made capable of feeling only in proportion as they have nervous branches distributed among them. 3. The excitement to all voluntary motion, or to those actions which are produced by the will, flows from the brain or spinal marrow, through the medium of the nerves, to those parts of the body which we wish to move. 4. The nerves are the organs, and the brain the receptacle of all our sensations, the source of all our ideas; from which it has been inferred that brain is the seat of the soul.

CHAP. XXVII.

OF MAN, HIS STRUCTURE AND FUNCTIONS,

Continued.

Organs of Sense-Seeing-Hearing-Smelling-Tasting-Touching. Organs of Circulation-Heart-Arteries-Veins-Absorbents-Glands, Organs of Respiration-Trachea-lungs-diaphragm. Organs of DigestionStomach-Intestines-Liver, and Pancreas-Digestion as a function: The Kidneys-Integuments-Cellular Membrane and Fat. The Skin

Perspiration-Hair-Nails.

OF THE EYE. The organ of vision, or the eye, is lodged, for its safety, in a socket formed partly by the bones of the skull, and partly by those of the face; and for the still greater security of this delicate organ, it is defended on the outside by the eye-lids, which serve as an occasional covering against external bodies; while a fine fluid, secreted or separated from a small gland, which is situated near the outer angle of the eye-lids, is constantly spread over the surface of the eye to keep it moist and transparent; and to wash away those particles, which, floating in the air, might produce injurious effects. This fluid, known by the name of tears, afterwards passes off by two small openings, at the opposite or inner augle of the eye, and thence descends by means of a canal,