Velocity of Muscular Action.-The velocity with which the voluntary muscles contract, depends in a great measure on the influence of the will, at the same time that it is regulated to a certain extent by habit, those movements to which we are accustomed being executed with the greatest celerity and ease, while those that practice has not rendered familiar are performed slowly and deliberately. The velocity of movement, as well as the influence of habit, are displayed by performers on musical instruments, as the violin, flute, and piano-forte, in writing, speaking, running, and so forth. The celebrated Haller states that he could articulate distinctly 1500 letters in the space of a minute; and as the relaxation of a muscle occupies as much time as its contraction, each change in the pronunciation of one of the letters in this instance must have occupied not more than the three thousandth part of a minute; yet this is slow in comparison to the velocity with which changes take place in drawing a straight line with the hand, where the line is composed entirely of points belonging to the circumference of as many circles, and where every point, of which there may be thousands in a second of time, requires an alteration in the condition of the muscles employed. Haller calculates that the rectus, one of the principal muscles of the thigh for the extension of the knee, contracts three inches in the twenty-eighth part of a second in the most rapid motions of the leg.

Though the human body affords numerous instances of quickness of motion almost inconceivable, yet the various classes of animals present many examples of velocity far surpassing any thing that occurs in the human subject. Eclipse, one of the fleetest race-horses on record, passed over a mile in a minute and a half, and Childers ran ninety feet in one second, which is at the rate of upwards of a mile in a minute. Haller has calculated that a race horse, at the height of his speed, must lift his leg in the seventieth part of a second. Still this is far outstripped

by what may be observed in birds, many of which are capable of wheeling round and round the most rapid racer in circles of immense diameter. Montagu, the ornithologist, estimates the flight of several of the falcon tribe as equal to 150 miles an hour. A falcon belonging to Henry IV of France made its escape from Fontainbleau, and was caught twenty-four hours after at Malta, a distance of not less than 1350 miles, being at the rate of about fifty-seven miles an hour; but as falcons do not fly by night, nor is it likely that he was caught immediately on landing at Malta, the flight must have been at a much greater velocity. Many other instances are afforded by birds, not merely of quickness of motion, but of long continuance and great power, not only without fatigue, but apparently with the greatest degree of enjoyment.

In our most rapid movements in travelling, we are often accompanied with insects making wide circuits round us, and that too against the wind. How incalculable, then, must be the celerity of motion in the muscles, that keep their wings in action! In force, how far comparatively do the muscles of the grasshopper or common flea, in making their leaps, surpass the gigantic strength of the elephant, or the energy of muscular contraction in the boa constrictor, when he crushes the bones of a large animal like the stag in his dreadful coils ! What duration of action is displayed in the muscles of the limbs of the sloth, which with ease clings to the highest branches of the loftiest trees when they are lashed with the fury of the hurricane! and yet that animal has been made the object of ignorant contempt and pity, as being doomed to an imperfection in the very organization where peculiar excellency of adaptation is displayed.

The different apparatus of motion are invariably adapted in the most perfect manner to the habits and circumstances of every animal, and the construction, adjustment, and action in all their various forms and diversities equally

declare the wisdom, power, resources, and beneficent provisions of the Divine Author of all, presenting to the inquiring and intelligent mind a subject for contemplation and study that is quite inexhaustible. The organs of motion even in one animal, such as man, would require for their elucidation volumes; and as knowledge advances, new and interesting views will present themselves in this admirable mechanism.

In our examination of an animal structure, in no instance do we find anything approaching to inconsistency of structure with function, but in every case the very reverse. Invariably do we perceive the most happy adaptation to, and the most perfect accordance with each other. It is only when we misconceive the purpose for which the mechanism is intended, or when, in our ignorance, we attribute to it purposes for which it never was destined, that incongruity, error, and imperfection appear.

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CHAPTER XIII.

FŒTAL LIFE.

General Observations-Progress of the formation of the Egg in the Common Hen -The Molecule-All the other parts subservient to it-Incubation-Formation of the Chick in the different stages-Membranes surrounding it in the eggTable of periods of Gestation in several Mammalia-The Vesicle-Its PassageArrival in the Uterus-Changes it undergoes through the various periods of Gestation, and the Functions successively called into action-Respiration in the Foetus-Regulation of Temperature-Secretions poured into the Alimentary Canal-Circulation of the Blood of the Foetus-Milk-Principal Constituents of -Proportion of these in different Animals-Conclusion.

In a work with the objects of the present, where the various parts of the animal machinery are intended to be described, it is necessary that some notice be taken of the earlier stages in the growth and development of the body, and of the various successive steps of its organization, at least so far as these seem to have been satisfactorily ascertained. At the same time, it appears both inexpedient and uncalled for that we should enter upon the consideration of the phenomena connected with the commencement of animal existence, still less to discuss the numerous discordant opinions that have been entertained respecting this subject; nor is it requisite to state the reasons for avoiding such disquisitions on the present occasion, since they must be obvious to every one.

The term embryo is applied to the new being before it arrives at any considerable degree of development, for instance, anterior to the fifth month in the human subject;

that of foetus, in the more strict acceptation of the word, designating its subsequent condition, though both terms are loosely and frequently indiscriminately used.

From the facility and accuracy with which observations can be made as to the progress of formation of the chick in the egg at any period of incubation, our most important knowledge respecting the development of the new offspring has been chiefly derived from the examination of it at different periods. Similar observations, made at different periods of gestation in mammalia, shew that the character and succession of the phenomena are in all the essential points identical. For this purpose the egg of the common domestic fowl is that which for convenience is selected. In order, therefore, to convey some notion of the manner in which the animal machinery is unfolded, the history and structure of the egg, in the first place, may be examined, that we may more readily comprehend the various changes which take place in the short period of twenty-one days, being that required for the formation of a perfect chicken out of the materials that compose it, when it has been placed under the necessary favourable circumstances.

Within the cavity of the abdomen of the common hen, and attached to the middle of the back, a cluster of globular bodies is situated, these bodies varying in their size from that of the head of the smallest pin to the fullsized yelk. These are the first rudiments of the egg. The smaller yelks are colourless and transparent, or nearly so; the larger have a more or less intense degree of yellow tint according to their size. Upon the yelks, even before they are detached from the cluster, there may be perceived a minute spot of great importance, as being the centre from which the formation of the chick subsequently commences. It is known by the name of molecule. The membrane of the ovary, as the cluster of yelks is called, is abundantly supplied with blood. On the larger yelks, numerous blood-vessels are seen distributed.