1. A sensation is the perception of an impression made on some organ. By our sensations we receive a knowledge of what is passing within or without the body. They are accordingly divided into internal and external, An internal sensation, is one which is produced by causes acting within the system, as hunger and thirst; but an external sensation is one that is occasioned by the impression of a body, external to the part impressed, as sight, hearing, etc.

2. No sensation, we have seen, can be perceived unless it is transmitted to the brain. When ideas, which have been called " the images of sensible objects," are reflected upon and compared with each other, we exert thought and judgment ; and when they are recalled, we are said to exert the memory. Thus are the senses avenues to knowledge, though they do not give rise, as some have stated, to our intellect and moral powers. With every sense an animal discovers a new world ; thus creation is to it increased or diminished, accordingly as its senses are more or less numerous. A sensation lasts a certain time after the exciting cause has ceased. Thus if a piece of wood, with one end ignited is whirled round, we see a luminous ring ; the sensation produced by the wood in each point of the circle continuing till the wood arrives at that point again ; a rocket forms a train.

3. The external senses are jive in number; touch, taste, smell, hearing, and vision. These are all situated at the surface of the body, so as to be capable of acting on external bodies. Most of them are under the control of the will; at least they may be exercised actively or passively, and by directing the attention, the impression may be rendered more vivid.

4. The sense of touch is the general feeling or sensibility produced by the skin, and which instructs us regarding the general qualities of bodies. It resides also to some extent in mucous membranes, as in the mouth, throat, etc, although in a very imperfect degree. For example, we cannot feel the pulse by placing the tongue over it on the wrist, as any one can ascertain by trying the experiment. The sense of touch is distinguished by some writers from that of feeling, but they are only modifications of the same sense.

5. I have already given some account of the structure of the skin, and stated that it is divided into four layers, viz., the cuticle, mucous web, papillary body, and true skin; and that the cuticle is the thin, transparent pellicle, or membrane, which forms the external incrustment of the body, protecting those papillae, in which reside the sense of touch. As no blood vessels or nerves have been traced in it, it is supposed to be an unorganized texture, or an albuminous secretion, poured out by the subjacent vessels, and hardened by the in fluence of the air. This accounts for its rapid formation, when destroyed by a blister, burn, etc. When a part of the body is exposed to irritation, the vessels which pour out the lymph, from which the cuticle is derived, have their action increased, and the cuticle either becomes thickened, as in the palms of the hand and soles of the feet, or the effusion is so abundant as to cause a separation between it and the true skin, causing blisters.

6. Now, it is a general law, that pressure promotes absorption ; and we see this exemplified daily in cases of tumours, abscesses, bandaging for dropsical effusions, etc. Had this tissue been subjected to this general law, the friction and pressure to which the cuticle of the hands is exposed in manual labour, and of the feet in walking, would, in a short time, have entirely deprived them of protection, instead of rendering their covering more dense and thicker, that it may more effectually serve as a shield to the parts it covers. As the cuticle is constantly renewed from its internal surface, it is also constantly peeling off in the form of a fine powder, or thin scales. After scarlet fever, and other diseases, attended with great heat of the skin, it is entirely removed ; the old scarf skin, as it is called, being thrown off in large patches. We find a cuticle in all organized beings, plants, as well as animals, though it differs greatly in structure and appearance. In many reptiles and crustaceous animals it is entirely shed at certain periods, presenting an exact mould of their bodies, their scales, and other external parts, and even their eyes being exactly represented.

7. The mucous web has already been pretty fully described. It is a soft pulpy net work, and seems to consist chiefly of the shaggy extremities of blood vessels, interlaced and bound together by delicate filaments of cellular membrane. It i& the seat of those minute globules which constitute the colouring matter which vary in their tints in different races. These tints are influenced in a greater or less degree by climate, for we find in tropical countries the colours of both plants and animals are more intense and brilliant than in colder regions, and that exclusion from light produces a pale blanched appearance, while exposure to it has a contrary tendency ; but these variations are neither permanent nor do they descend to the offspring.

8. The dermis, or true skin, is the thickest part of the external integuments, and is composed of an infinite number of plates, consisting of filaments inextricably wound together, and abundantly supplied with blood vessels and nerves. The external surface of the cutis is every where studded by very minute nipples or papillae, and in several parts, as the palms of the hands and the ends of the fingers, they are arranged in symmetrical rows, which form wavering lines, and separated by small crevices that admit of the flexions of the skin and its adaptation to the surfaces of external objects. These papillae are the terminations of nerves and blood vessels, and are the immediate instruments of touch. Although covered by the cuticle and mucous web, yet the removal of these by blisters or otherwise, does not increase the sense, but tends to destroy and disturb it. From this arrangement, sensation is communicated through the hair and nails, as well as the cuticle. In animals, of the cat tribe, which are furnished with whiskers, these serve as organs of touch, as their roots terminate in these nervous papillae.