animals; for, when we consider the cold medium in which they live, it must be obvious, that were their adipose matter as easily congealed as that of the sheep or ox, much inconvenience might be the result. In the seal tribe, the walrus, the manati, and other webfooted mammalia, it is equally fluid, and equally resists the cold of the water and of the frozen regions which those animals inhabit.

Fat is a bad conductor of heat, and hence the thick coating of blubber which invests the whale tribe prevents their animal caloric from escaping, and thus enables them to endure the utmost severities of winter. But it serves to them also another very important purpose, namely, that of enabling them to keep at the surface of the water, with a small exertion of muscular power; a matter of the utmost importance to animals which must rise to breathe air, as man and other hot-blooded animals do, by lungs, and not like fishes, which act upon the air contained in water by gills. The great mass of light blubber makes them nearly equiponderate with the water, and enables them to rise to the surface to breathe with little effort, and what is not less important, keeps them at the surface during sleep.

In the cachalots or spermaceti whales, the provision for this last state is even still more evident. Their head is of a vast size, and within it are enormous chambers filled with the light substance called spermaceti, which, mixed with oil, gives their head such a buoyancy, that when the animal is at rest or sleeping, it resembles a rock or small island emerging from the sea. To this circumstance, we may trace the fabulous accounts detailed by Gesner and other old naturalists, of sailors disembarking on the back of a whale, mistaking it for an island, and not aware of their situation till the huge animal, annoyed by their presence or pained by the fire they had kindled, began to move and plunge into the deep. To these fictions we are indebted for the following exquisite lines of Milton, which are not less beautiful, perhaps, as poetry, that the epithet "scaly rind" is incorrect:

"Him haply slumbering on the Norway foam,
The pilot of some small, night-foundered skiff
Deeming some island, oft, as seamen tell,
With fixed anchor in his scaly rind,

Moors by his side, under the lee, while night
Invests the sea and wished morn delays."

G

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CHAPTER IV.

OF THE MUSCLES.

You know what beef is; that it is the red flesh of an ox. Well, then, beef is muscle; but seeing it as you have been accustomed to do, will give no more an adequate idea of what a muscle is, than a heap of dilapidated walls and rubbish could give of the beauty and regularity of the temple or building of which they previously had formed a part. Though beef is red, muscle is not always nor necessarily of that colour, for you know that, in general, the flesh, or in other words, the muscles of fishes, are white; in many birds it is nearly white also, and in various parts of the human body the muscular fibre is whitish grey, as in the muscular coat of the stomach.

The bones may be considered as passive materials in our composition, but the muscles are the grand instruments of animal motion; you cannot advance a single step, nor lift your hand to your mouth; you can neither open nor shut the latter, nor raise your eye to heaven,

MUSCULAR FIBRES.

75

nor bend it to earth, but through the action of muscles: in short, all animal motions, a few excepted, depending on what is named the erectile tissue, are performed by muscular contraction.

But, before proceeding farther, I must give you some idea of the structure of these moving organs. First, then, the chief bulk of a muscle is composed of certain fibres or threads, connected to each other by cellular membrane, and I cannot now, perhaps, give you a better general idea of this kind of fibre, and its connecting medium, than by directing you to a very humble illustration; namely, the examination of a bit of boiled hung, or smoked beef. You know that this can be easily separated into threads, and in so doing, you will observe that as each fibre is torn from another, you bring into view a white cottony substance, which had joined the separated thread to its fellows. The thread is the muscular fibre, and the cottony medium is the cellular substance.

In this simple way you recognise these two components of a muscle; but you must not stop here, examine closely any one of the detached threads, and you will find that it may be divided

into still smaller threads or fibres, and when you have arrived at a fibre, which, to your naked eye, appears absolutely single, use a magnifying glass, and you will discover that even this apparently single part is not simple but compound, that is, composed of fibres still smaller.

Having gone so far in your examination, you will be aware that the ultimate muscular fibre must be extremely minute, though, perhaps, you will not go so far as Haller in supposing that it can only be seen cum acie mentis, which is equivalent to saying, that it cannot be seen at all. Nothing is more certain, however, than that the ultimate muscular fibres are wonderfully minute; as a proof of which I may mention, that Lieuwenhoeck counted in one fibre 3181 fibrillæ or smaller fibres, and that Hook found fibres in the claw of a crayfish, five hundred times smaller than a human hair.

You will remember, therefore, that every fibre which you can perceive in a muscle is compounded of many fibrillæ, or smaller fibres; and, now, we must consider how these are united to form a muscle, strictly so called. When we examine, for instance, the deltoid