But it is in the motions of the trunk that the abdominal muscles are called most into play. In all the inflexions of the trunk, whe ther the body be horizontal or erect, these muscles are main agents. When the body is recumbent on a horizontal plane, the recti are thrown into action when the individual attempts to raise up the thorax, the spine being thereby brought into the state of flexion. If the thorax be fixed, while the body is still supine, the action of the recti will draw the pelvis upwards and forwards, causing slight flexion of the spine, and slightly approximating the upper margin of the pelvis to the lower margin of the thorax.
Although the real muscles are the principal agents in thus flexing the spine, the obliqui co operate with them very powerfully, and are especially useful in maintaining the due propor tion between the middle and lateral regions of the abdomen. When the two obliqui of the same side act together, the direction of their force is, as with all oblique muscles whose fibres decussate, in the diagonal between their fibres; and, therefore, when the obliqui of op posite sides act in unison, they very powerfully aid the recti in flexion of the spine, approx imating the thorax and pelvis anteriorly. When the obliqui of one side act, they produce a lateral inflexion of the trunk to that side,—the middle and opposite region of the abdomen being in this position rendered prominent by the viscera pushed over from the side of the contracted muscles. In what have been called the rotatory motions of the trunk, the obliqui muscles of the same side antagonize each other; thus in that movement by which the anterior surface of the trunk is made to look to the left side, the obliquus externus of the right side will co-operate with the obliquus internus of the left, but the obliquus internus of the right will antagonize the external muscle of the same side. " These muscles," ( obliquiexterni et in terni,) says Dr. Barclay, " from occupying the whole of the lateral aspects extending be tween the ilia and ribs, and from acting at the greatest lateral distance from the centre of motion, must always be muscles principally concerned in producing inflexions dextrad and sinistrad on the lumbar vertebrae, principal di rectors in all the inflexions sternad and dorsad; and, from the assistance which they give to the recti, principal librators also of the trunk, whe ther we be sitting, standing, or walking."
The reciprocal action of the recti and ob liqui on each other is one of the most beauti ful parts of the mechanism of the abdominal muscles. This is mainly to be attributed to the close connection which subsists between these muscles in consequence of the formation of the sheaths of the recti by their aponeuroses, and the adhesion of the anterior wall of those sheaths to the tendinous intersections of the recti. When the recti contract, the antero-pos terior diameter of the abdomen is diminished, and consequently the viscera are pushed to wards the sides ; when, on the other hand, the obliqui contract, they diminish the transverse diameter of the abdomen, and push the viscera forward in the middle line. In the one case, then, it will be evident that the obliqui act as moderators to the recti, and in the other the resistance of the recti moderates the action of obliqui,—the former muscles being, as Cru veilhier remarks, as it were, two active pillars compressing forcibly the viscera against the an terior surface of the spine. It is probably to enable the recti to act more completely as moderators upon the several segments of the obliqui that they are intersected by tendinous lines, with which the aponeuroses of those muscles are connected. Another use has been assigned to these intersections by Bertin, viz.,—to multi ply the points of attachment of the obliqui muscles, and to associate them, in many ac tions, with the recti muscles. This is explained by a reference to the action of the recti in flex ing the pelvis : were these muscles uncon nected with the obliqui, they would act only on the pelvis, into which they are inserted; but in consequence of the insertion of the internal oblique into the intersections of the recti, and the attachment of that muscle also to the crista ilii, the force of contraction of the recti is com municated not only to the pubis, but also through the fibres of the obliquus internus to the rest of the pelvic margin.* The action of the pyramidales seems to be chiefly on the linea alha, which they render tense ; thus limiting the separation of the recti, and opposing the tendency to visceral protru sion. Fallopius supposed that they acted on the bladder, especially when it was in a dis tended state ; and Parsons conjectured that they might depress the suspensory ligament of the bladder (the urachus), and thus facilitate the contraction of that organ.