immediately beneath the great coal-field, is incorrect, it being only "an alternating series lying above the great body of carboniferous limestone. This objection is obvious, if what I have termed the transition limestone at the base of this series be secondary, "carboniferous;" and if it can be proved, in the district I have imperfectly described, to "repose conformably on the extensive formation of old red sandstone."

On this head I wish to be understood, not as insisting on the infallibility of my own individual sentiments, but as desirous of apprising geologists of the grounds on which those views were founded. At the same time I am not quite prepared to admit, with Mr. Weaver, that the whole of the coal deposits in Pennsylvania, belong to one great carboniferous order, and that the fact" is fully established by Professor Eaton." (p. 131.)

A vast deal of investigation remains yet to be entered into before these debateable points can be adjusted. There is good prospect of some of them being shortly elucidated by the geological surveys simultaneously going on in the States of New York, Pennsylvania, Maryland and Virginia. Philadelphia, Sept. 23, 1836.

LXXVIII. Physiological Remarks on certain Muscles of the Upper Extremity, especially on the Pectoralis Major. By F. O. WARD, Esq., King's College, London.*

[With a Plate.]

THERE is a remarkable fold in the tendon of the pectoralis major, which, though described by all anatomists, has never yet, I believe, been explained. The muscle consists of two portions, one smaller and upper, arising from the clavicle, and passing downward and outward; the other larger and lower, arising from the sternum and ribs, and having a general direction upward and outward. The fibres of the muscle thus converging towards each other, terminate in a flat tendon several inches wide, which is attached to the upper part of the humerus.

Instead, however, of having the usual simple insertion represented in Plate IV. fig. 1, the lower part of this tendon is folded up, behind the upper portion, so that the margin B appears above the margin A, as represented in fig. 2.

As it is an axiom in physiology that every arrangement is to be accounted for, this peculiar twist has given rise to several speculations. Some suppose it designed merely to diminish the extent of the insertion. Others believe it to have

* Read before the Royal Society June 16th, 1836: and now communicated by the Author.

the effect of equalizing the length of the muscular fibres. But independently of the consideration that the insertion would have been quite as compact if the tendon had been thick and single, instead of thin and double; and that the fibres are not by any means equal in length, according to the second hypothesis, both explanations are defective, in as much as they show no reason for the muscular fibres crossing each other, so that the upper are attached below, and the lower above, the medium point of the whole insertion.

I think that the arrangement becomes perfectly intelligible when the separate actions of the upper and lower portions of the muscle are considered with reference to the species of motion those actions require.

The separate action of the lower fibres is to depress the arm when raised; that of the upper fibres, to raise the arm when depressed. Of this any person may convince himself by laying the hand on the muscle; first, while imitating the action of hammering; and then, while raising or supporting a weight: in the former case he will perceive a momentary convulsive contraction of the lower fibres; and, in the latter, a steady, continued tension of the upper.

In the third and fourth sketches which exhibit these positions, the several directions of the humerus, and of the upper and lower portions of the muscle, are represented by lines, the arrow-head denoting in each figure which set is exerted, and in what direction it acts.

Now since the humerus is a lever having the fulcrum at one end and the resistance at the other, the velocity it acquires must be directly, and the force inversely, proportionate to the proximity of the moving power to the fulcrum.

The most common, and therefore most important purpose, to which the depressing fibres are applied, is that of bringing down the arm in using the hammer, pickaxe, &c., as the carpenter, blacksmith, goldbeater, and a hundred other artizans testify. In these motions velocity alone is required from the muscle, the gravity of the tool giving force to the blow; and to produce this velocity the lower division is attached near to the fulcrum. Again, the commonest employment of the upper fibres consists in such actions as lifting, drawing, and the like, in which force, not velocity, is the desideratum; and, in order to obtain force at the expense of velocity, the insertion of these fibres is brought down as far as possible towards the resistance.

It is remarkable that, in each instance, that very fasciculus. of the muscle, which possesses most of the action peculiar to its division, possesses likewise that very point of the insertion

which affords it most of the leverage it requires. Thus it is not the uppermost portion, a, of the elevating division, (see the Italic letters in figs. 2 and 4,) which is attached to the lowest point, D, of the insertion, because there are succeeding fibres (as b, figs. 2 and 4,) which form a less acute angle with the humerus while depressed: whereas it is the lowermost fasciculus (C, fig. 2,) of the lower division, that seeks the highest point, B, of the insertion, because this portion forms the least acute angle with the humerus when elevated (see fig. 3). This trait adds another to the innumerable proofs of the minute accuracy of the animal organization.

Any action which requires from either portion, a species of motion contrary to that which it is adapted to produce, -as raising the body by the hands, which requires force from the fibres of velocity,-soon fatigues the muscle. Turning a winch, which is another instance of the same kind, is notoriously a very disadvantageous application of human strength; and any employment in which steady and forcible pushing has to be performed by the arms raised above the head, is extremely fatiguing. In throwing a heavy quoit, which requires both accuracy and force, the arm is swung by the side; but in throwing a light ball, for which velocity is requisite, the arm is always swung above the head. Cricketers are practically such good physiologists in this respect, that they have enacted a law which compels the bowler to swing his arm by his side in throwing the ball,-because, if the ball were flung "overhanded" at the wicket, from so near a point as the bowler's station, its velocity would be unmanageable; whereas the "long-throw" who has to send up the ball from a distance, always swings his arm above his head.

The muscles associated with each division of the pectoralis major bear out the proposed explanation by the analogy of their insertions. Thus the coraco-brachialis, and the anterior fibres of the deltoid, which cooperate with the upper division of the pectoralis major, are attached to the front of the humerus, half-way down; evidently for the purpose of gaining force, which they do want, at the sacrifice of velocity, which they do not.

On the contrary, the teres major and latissimus dorsi, which assist the lower division of the pectoralis major in depressing the humerus, act, like that muscle, near the fulcrum of the lever; being attached to the inner margin of the bicipital groove, just opposite to the pectoralis tendon. These two muscles, indeed, are in several respects analogous to the two divisions of the pectoralis major. The teres major, which is superior and smaller, and arises from the scapula, may be

compared to that portion of the pectoral which is superior and smaller, and arises from the clavicle; and the latissimus dorsi which is inferior and larger and arises from the vertebræ and ribs, resembles that portion of the pectoralis major which is inferior and larger, and arises from the sternum and ribs. The tendons of the two dorsal, like those of the two pectoral muscles, are continuous at their lower margins, and, as if to render the analogy complete, (though, in fact, to render the leverage suitable,) the lowest fibres of the latissimus dorsi are folded around the teres major, and inserted above it into the humerus; because they are most nearly at right angles with the bone when lifted to strike, and therefore most effective in drawing it down. Fig. 5 is a front view of the insertion of these two muscles, B representing the teres major, C the latissimus dorsi folding round it to gain a higher point of attachment, A the tendon of the pectoralis major raised out of its natural position, and D the bicipital groove to the borders of which these muscles are attached. The proposed explanation is further borne out by the comparative anatomy of the pectoral muscle in birds, in which it is developed to a very large size on account of being the principal motor of the wing. In these animals there is no crossing of the fibres of the pectoralis; they all assist in performing one action, and are consequently inserted in regular order, those which are superior at their origin having also a superior insertion, and vice versâ, as may be seen in fig. 6, which is a sketch of the pectoral muscle of a pigeon. The turning under of the fibres represented at a seems at first sight to indicate a decussation of the upper and lower portions of this muscle, similar to that which occurs in the corresponding organ of man. But the resemblance disappears when the muscle is divided along the dotted line bc, and the humeral portion reflected as in fig. 7. It then becomes evident that the lower fasciculi though forming a little bundle partly distinguishable from the rest of the muscle, and inserted by a separate slip of tendon, nevertheless join the bone below the upper fasciculi, and below the central point of the whole insertion. Professor Rymer Jones, who very kindly examined with me the muscles of the breast in the pigeon, confirms the accuracy of this observation.

There is, however, an action, which, as it furnishes man with his most obvious means of self-protection, must have been carefully provided for by Nature, and which seems to throw doubt on the correctness of the foregoing explanation. I mean the action of throwing the extremity forward, as in boxing. In this action, which requires great velocity, although all the fibres of the pectoralis major are in some measure

brought into play, the upper set, that namely of least velocity, are, it must be admitted, the principal agents, so far as this muscle is concerned in other words, Nature, according to my explanation, causes a muscle to work at disadvantage, in an action of essential importance.

This, I think, is only an apparent difficulty, for in this motion as correct a balancing of leverage is displayed, as can anywhere be found throughout the body.

The fist is thrown forward by a double motion. The humerus, represented by A B, fig. 8, revolves round the point A till it takes the position A C, while the forearm, represented by BD, revolves round the point B till it takes the position B E, so that the resulting position of the whole extremity is ACF. The upper division of the pectoralis major, the anterior fibres of the deltoid, and the coraco-brachialis, are the main causes of the first motion; the triceps, anconæus, and supinator muscles, of the second.

The distance which the forearm passes through, represented by the curve 2, exceeds considerably the space traversed by the upper arm, represented by the curve 1; but as the motion of the forearm round the point B is from above downwards, its extensors have no weight to raise; on the contrary, are assisted by gravity. Whereas the humerus, though it moves through a shorter distance, moves upward, and carries with it the forearm, so that its elevators have to raise a considerable weight. In order that these two motions may be completed in the same time, the former requires the greater velocity, the latter the greater force. Accordingly the triceps and its associate extensors, act on the ulna by a lever between one and two inches long, while the three elevators of the humerus act by levers whose respective lengths are about four, five, and six inches. See figs. 8 and 9, in which P represents the tendon of the pectoralis major, D that of the deltoid, C that of the coraco-brachialis, and T that of the triceps.

I may just add, (for it is interesting to observe the unconscious acquaintance which every man has gradually acquired with the precise capabilities and most effective application of every fibre in that complicated machine, his own frame,) that in preparing to strike a blow the elbow never hangs close to the side, as in fig. 11, but is always thrown out, as in fig. 12; in order that the elevator muscles, all of which draw more or less inward, as well as upward and forward, may act during the strong effort at their full advantage.

Thus, then, not only is the leverage of the upper and lower portions of the pectoralis major accurately adapted to the ac