18 



ABDOMEN. 



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. 



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 recti 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," (obliqui externi 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- 



* Barclay on Muscular Motion, p. 522. 



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 Berlin, 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 alba, 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. 



The other muscles which are from situation 

 abdominal muscles in consequence of their 

 connexion with the posterior wall of the abdo- 

 men, are chiefly agents in the extension of the 

 vertebral column : in their contracted state, 

 however, they form a tense and resisting sur- 

 face, against which the viscera are compressed 

 by the contraction of the anterior muscles. 



II. Of the Abdominal Cavity. The annexed 

 engraving (fig. 5.) exhibits a view of the abdo- 

 minal cavity, the anterior and part of the lateral 

 walls having been cut away and the viscera 

 removed. The subject is so bent backwards 

 as to render the bodies of the vertebras very 



* Berard, loc. cit., ct Berlin, sur 1'usage des 

 enervations des muscles droits du bas-ventre, in 

 Mem. do 1'Acad. des Sciences de Paris. 



