248 MUSCLES OF THE ABDOMEN. 



arising from the lowest ribs, being directed nearly vertically upwards from their costal 

 attachments, must tend to pull those ribs upwards. The serratus posticus inferior 

 and quadratus lumborum muscles, by opposing the diaphragm, and thus giving it a 

 fixed point on which to descend, become assistant muscles of inspiration. The ante- 

 rior fibres of the diaphragm being directed more horizontally towards the central 

 tendon, oppose the forward motion of the sternum : hence the sternum becomes 

 arched in patients long subject to asthma. (Hutchinson, Article " Thorax," in Todd's 

 " Cyclopaedia of Anatomy and Physiology ; " Meyer, " Physiologische Anatomic.") 



In more forcible inspiration, and more especially in severe attacks of dyspnoea, 

 there are called into play other powerful muscles, to secure the inspiratory action of 

 which a fixed attachment must be provided by the support and elevation of the 

 shoulder and arm ; among these may be enumerated the serratus magnus and pec- 

 toralis minor, acting from the shoulder, and the pectoralis major and latissimus dorsi, 

 acting from the raised arm, which together produce expansion and elevation of the 

 ribs. 



Expiration. In normal and quiet expiration the diminution of the capacity of the 

 chest is mainly, if not wholly, due to the return of the walls of the chest to the condi- 

 tion of rest, in consequence of their own elastic re-action, and of the elasticity and weight 

 of the viscera and other parts displaced by inspiration ; the lungs themselves, after 

 distension by air, exert considerable elastic force, and no doubt the ribs and their 

 cartilages re-act strongly by their elastic return from the elevated and bent condition 

 into which they had been thrown by the inspiratory forces. In more forcible acts of 

 expiration, in muscular efforts of the limbs and trunk, and in efforts of expulsion 

 from the thoracic and abdominal cavities, all the muscles which tend to depress the 

 ribs, and those which compress the abdominal cavity, concur in powerful action to 

 empty the lungs, to fix the trunk, and to expel the contents of the abdominal viscera. 

 (See further " Action of the Abdominal Muscles.") 



MUSCLES OF THE ABDOMEN. 



The muscular wall of the abdomen is formed, in great part of its extent 

 on each side, of three layers of muscle, the fibres of which run in different 

 directions ; those of the two most superficial layers lying obliquely, and 

 those of the innermost layer being transverse. In front those three layers 

 of muscle are absent, and are replaced by tendinous expansions, which meet 

 in the middle line, and on either side of that line the fibres of the recti 

 muscles extend in a vertical direction between the tendinous layers, sup- 

 ported usually at the lower end by the pyramidales muscles. Posteriorly 

 the wall is formed in part by aponeurosia, and in part by muscles of which 

 the fibres are chiefly vertical, viz., the muscles of the back, and in front of 

 them the quadratus lumborum. 



The superficial or external oblique muscle (descending or gieat oblique) 

 arises from the outer surface of the eight inferior ribs, by slips arranged in 

 a serrated series, four or five of them meeting with origins of the serratus 

 magnus, and three or four with origins of the latissimus dorsi. The slips of 

 these two sets of muscles alternate with each other, as the tips of the fingers 

 of one hand may be made to fit in between those of the other, and hence 

 they are said to interdigitate, and are termed digitations. The lower and 

 the upper digitations of the external oblique are connected with the ribs near 

 their cartilages, the others are attached to the ribs at some distance from 

 their extremities ; the lowest digitation generally embraces the point of the 

 twelfth rib. The fleshy fibres from the last ribs pass down in nearly a 

 vertical direction to be inserted into the external margin of the crest of the 

 ilium for about the anterior half of its length ; all the rest incline down- 

 wards and forwards, and terminate in tendinous fibres, which form the 

 broad aponeurosis by which the muscle is inserted. 



