MUSCULAR SYSTEM. 121 



and second of these vertebrae. The two appendices form a union 

 and decussation opposite to the seventeenth dorsal vertebra; and 

 afterwards, again bifurcate. The greater and lesser muscles form 

 a conjunction through the medium of the cordiform tendon. 



Relations. — The anterior or convex surface is covered by the 

 pleura; is opposed to the bases of the lungs, and in part to the 

 sides of the chest; and is connected, next to the spine, with the 

 superior mediastinum and its important contents; next to the 

 sternum, with the pericardium and inferior mediastinum. The 

 posterior or concave surface is covered by peritoneum ; and is, 

 together with the crura, connected, superiorly, with the psoas 

 muscles and quadratus lumBorum ; infero-posteriorly, with the 

 kidneys, renal capsules, pancreas and duodenum ; on the right 

 side, with the liver; on the left, with the stomach and spleen. 



Direcfion. — The fibres of the larger muscle all converge in 

 radii, and run to the cordiform tendon, as to a common centre: 

 those of the appendices run longitudinally. 



<itnicture. — It is made up of fleshy and tendinous parts. 

 The fleshy parts are those forming the circumferent portions of 

 the large muscle, and the principal bulk of the crura or appen- 

 dices. The tendinous parts consist in a thin circular expansion 

 occupying the middle of the larger muscle, and uniting that with 

 the lesser. Through the muscle are to be seen three remarkable 

 openings : a superior one, in the interspace between the crura, 

 for the aorta ; one, a little lower, formed by the decussation of 

 the crura, for the oesophagus ; and a third, or lower one, perforat- 

 ing the cordiform tendon, for the vena cava posterior. 



Action. — The diaphragm is the principal if not the sole agent 

 in inspiration : it acts in opposition to the abdominal muscles, 

 which are the chief expiratory powers. By the contraction of its 

 radiated fibres, aided by that of the crura, the cordiform tendon 

 is transformed to a plane surface, and the dimensions of the 

 chest from before backwards thereby considerably augmented. 

 Its capacity, however, is again contracted in consequence of the 

 muscle resuming its concavo-convex form, from the reaction of 

 the abdominal muscles and consequent pressure against it of the 

 abdominal viscera. There are occasions, however, in which these 

 opponent respiratory agents co-operate to one and the same effect; 

 such is the case in the expulsion of urinary and facal matters ; 

 and also in the act of parturition. 



