The consideration of the action and uses of the abdominal muscles naturally comes under two heads : —1. their action upon the abdo minal cavity and its contents ; 2. their influ ence on the trunk generally, or parts of it.
It is the muscles that enter into the compo sition of the 'anterior and lateral walls of the abdomen which act chiefly on the cavity'and its contained viscera. The solidity of a con siderable portion of the posterior wall, and the great strength of the lumbar muscles, give to that wall such a power of resistance as enables it to receive the compressed viscera without at all yielding. A reference simply to the attach ments of the muscles of the anterior and lateral walls is sufficient to shew that these muscles when contracted must diminish the capacity of the abdomen, both in the lateral and antero posterior directions ; and as the posterior wall is but little influenced, the viscera will be pushed partly upwards against the diaphragm, and partly downwards into the cavity of the pelvis, where their further descent is opposed by the levator ani. Hence it appears that a degree of antagonism exists between the diaphragm and the abdominal muscles, as well as also between those muscles and the levator ani. It is extremely difficult to maintain the abdominal muscles and the diaphragm at the same moment in a state of contrac tion ; in general they alternately yield the one to the other : and when it does happen that they are simultaneously contracted, the abdominal viscera must suffer an unusual de gree of compression ; and it is not improbable that vomiting is sometimes produced by such a cause, and defecation, no doubt, is likewise aided by it. The danger of the protrusion of some of the hollow viscera between the fibres of, the muscles is provided against by the variation of direction in the fibres of the several layers ; thus the fibres of the obliqui are in the directions of two intersecting diagonals, and those of the transversalis are different from both. By this arrangement a sort of network is formed, with meshes so small as to render a protrusion perfectly impossible in the healthy condition of the muscle. In the compression of the viscera the abdominal muscles are most completely congeneres, although the trans versalis seems to be the best adapted to this action, and probably, for that reason, forms the layer which is placed nearest the peri toneum. The recti muscles are powerful auxiliaries in affording a fixed point of attach ment in front for the aponeuroses of the broad muscles, and the pyramidales assist in a similar manner by rendering tense the Linea alba. Is this constant action of the abdominal parietes on the viscera necessary or favourable to the due performance of the functions of those organs, or to the continuance of the abdominal circulation ? There certainly does not appear to be any evidence for the necessity of them for this purpose : that they are favour able to it may be inferred from the fact that they do bear their present relation to them. We know from numerous experiments on animals, that both the transmission of the intestinal contents, and the abdominal circulation may go on when the abdominal muscles have been freely opened or removed. Hence we may answer this question with perfect justice in the words of Bichat : " The walls of the ab domen favour these functions by their motions; but these motions are by no means essential to them." It is in consequence of the power which the abdominal muscles thus appear to exert in compressing the viscera, that some physiolo gists have attributed the act of vomiting to their action united with that of the diaphragm; and Magendie, reviving the opinions of Bayle, Chi rac, and Shwartz,* went so far as to deny to the muscular coat of the stomach any partici pation in this act, and to ascribe it wholly to the influence of the abdominal muscles. But Beclard, to whom the question was referred by the Academy of Medicine of Paris, proved satisfactorily that the abdominal muscles are active in producing vomiting when the sto mach is distended in a certain degree, and that the muscular coat of the stomach is also active in emptying the contents of that viscus. This
conclusion Haller had arrived at long ago, and clearly expresses it in the following passage : " Evidentissimum ergo videtur, vomitus qui dem causam esse in ventriculo eumque in con tractionem niti propriis viribus atque aliquando vomitum perficere. Plerumque tames irrita tionem in ventriculo natam et sensum summw anxietatis, quae vomitum prtecedunt, facere ut ad levandam wgrimoniam vines diaphragmatis et musculorum abdominis excitatm atque mo lestiam de homine amoliturze, vomitum per ficiant. Unde neque a sola voluntate in ple risque certe mortalibus vomitus cieri potent neque a scat absque voluntate natura—Quare recte conjunctas vines ventriculi et organoruin respirationis Cl. Viri fecerunt. Et videtur dia phragms et abdomen plus virium habere, quando ventriculus aut cibis repletus est, aut clausis ostiis distentus : tune enim magis ad perpen diculum proximum ventriculum cornprimunt et tota contingunt." If it be admitted that the abdominal muscles are active in producing vomiting, and in defe cation and micturition, it will follow likewise that they must assist in parturition. While these pages were preparing for press, the fol lowing passage presented itself to me, in an able and interesting review of M. Velpeau's Treatise on Midwifery. It so fully illustrates the part which the abdominal muscles take in promoting parturition, that I venture to tran scribe it, " It is certain," says the reviewer, " that a woman who bears down' as it is termed, with all her force, who makes the most of her pains, however feeble they may be, will thus accelerate her delivery ; and that another may more or less delay delivery by voluntarily opposing muscular action as much as she can. For example ;—a woman was admitted for de livery at M. Baudelocque's theatre ; labour went on regularly, and the pupils assembled. The dilatation of the cervix now slackened, and no progress was made during the whole night. The were fatigued and retired ; the pains immediately returned, and dilatation again went on. The young men again entered ; the phenomena of labour again ceased. Baude locque, suspecting the cause, gave a hint to the students to retire, but to be at hand and enter upon a given signal. The patient now began to bear down,' and the head of the child was quickly at the vulva. The spectators were once more brought to the scene of action, and the labour was speedily terminated; for it had now advanced too far to be suspended by any voluntary effort or moral alarm of the woman."-f The fixedness of the inferior attachment of the abdominal muscles to the pelvis, and the mobility of the ribs, to which they are attached superiorly, evidently indicate that these muscles are destined to act upon the thoracic cavity. The transversalis does not, from the direction of its fibres, admit of this action to any extent ; that office, therefore, devolves chiefly on the obliqui and recti. When these last-named muscles act together, they must compress the inferior opening of the thorax, draw its inferior margin downwards and backwards, and, by the compression thus exerted on the abdominal vis cera, push them upwards against the diaphragm, which muscle is thus made to ascend into the thorax, and that cavity is thereby diminished in its vertical and antero-posterior diameters, and also, though not so obviously, in its trans verse. Hence the lungs become so compressed as to be adapted to the altered capacity of the thorax, and thus these muscles must be con sidered as very important agents in the act of expiration. It must be observed, however, that in order that they may act on the chest with their full force, it is necessary that that cavity should have been previously in a state of full dilatation, for under such circumstances the fibres of the obliqui and recti are con siderably stretched and their levers elongated.* It is in the excited states of expiration, cough ing, sneezing, &c., that this action of these muscles is most obvious.