THE RESPIRATORY MSCSANI8M, 



381 



The Vertical Enlargement of the Thorax. This is 

 brought about by the contraction of the diaphragm which 

 (Figs. 1 and 128) is a thin muscular sheet, with a fibrous 

 membrane, serving as a tendon, in its centre. In rest, the 

 diaphragm is dome-shaped, its concavity being turned towards 

 the abdomen. From the tendon on the crown of the dome 

 striped muscular fibres radiate, downwards and outwards, to 

 all sides; and are fixed by their inferior ends to the lower 

 ribs, the breast-bone, and the vertebral column. In expiration 

 the lower lateral portions of the diaphragm lie close against 

 the chest-walls, no lung intervening between them. In in- 

 spiration the muscular fibres, shortening, flatten the dome 



FIG. 127. The skeleton of the thorax, a, g, vertebral column; 6, first rib; c, 

 clavicle; d, third rib; i, glenoid fossa. 



and enlarge the thoracic cavity at the expense of the ab- 

 dominal ; and at the same time its lateral portions are pulled 

 away from the chest-walls, leaving a space into which the 

 lower ends of the lungs expand. The contraction of the 

 diaphragm thus increases greatly the size of the thorax cham- 

 ber by adding to its lowest and widest part. 



The Dorso-Ventral Enlargement of the Thorax. The 

 ribs on the whole slope downwards from the vertebral 

 column to the breast-bone, the slope being most marked 

 in the lower ones. During inspiration the breast-bone 



