246 MUSCLES OF THE THORAX. 



Foramina. There are in the diaphragm three large perforations for the 

 passage respectively of the aorta, the oesophagus, and the vena cava, besides 

 some smaller holes or fissures which are less regular. a. The foramen 

 for the aorta (hiatus aorticus), placed in front of the vertebrae, is bounded 

 by tendinous fibres of the crura as already described. Besides the aorta, 

 this opening transmits the thoracic duct, and generally also the vena 

 azygos. b. The foramen for the oesophagus, higher and farther forward 

 than the preceding, as well as a little to its left, is separated from that 

 opening by the decussating fibres of the crura. It is oval in form, and is 

 generally entirely surrounded by muscular fibres ; in some rare cases, 

 however, a small part, the anterior margin, is found to be tendinous, being 

 formed by the margin of the central tendon. c. The opening for the vena 

 cava (foramen quadratam) is placed in the highest part of the diaphragm, 

 in the tendinous centre at the junction of the right and middle alee, 

 posteriorly. Its form is somewhat quadrangular ; and it is bounded by 

 fasciculi of tendinous fibres running parallel with its sides. 



Besides the foregoing large foramina, there are small perforations through the crura 

 for the sympathetic and splanchnic nerves on both sides, and for the vena azygos minor 

 on the left side. Moreover, the larger azygos vein often takes its course through the 

 right crus. 



The upper or thoracic surface of the diaphragm is highly arched. Its posterior and 

 lateral fibres, ascending from their connection with the lower margin of the thorax, 

 are for a considerable extent placed close to the ribs, the lungs not descending so far 

 as their attachments. The vault of the diaphragm rises higher on the right than on 

 the left side. On the right side in the dead body it rises to the level of the fifth rib 

 at the sternum, and on the left side only as high as the sixth. This difference has 

 relation to the great size and firmness of the liver on the right side. It is covered 

 superiorly by the pleura and the pericardium ; the fibrous layer of the latter mem- 

 brane blending with the tendinous centre, as well as with the fascia covering its 

 muscular substance. The lower surface, of a deeply concave form, is lined by the 

 peritoneum, and has in apposition with it the liver, the stomach, the pancreas and 

 spleen, and the kidneys. 



ACTIONS MOVEMENTS OP RESPIRATION. The mechanical act of respiration consists 

 of two sets of movements, viz., those of inspiration and of expiration, in which air 

 is successively drawn into the lungs and expelled from them by the alternate 

 increase and diminution of the thoracic cavity. The changes in the capacity of the 

 thorax are effected by the expansion and contraction of its lateral walls, called costal 

 respiration, and by the depression and elevation of the floor of the cavity, through 

 contraction and relaxation of the diaphragm, called abdominal respiration. These 

 two methods of respiration are normally combined, but in different circumstances 

 one method is resorted to more than another. Thus, abdominal respiration is most 

 employed in the male, costal respiration most in the female. 



Inspiration. The study of the movements of the thoracic walls in respiration 

 presents the difficulty that those movements cannot be perfectly imitated on the pre- 

 pared skeleton, because the force cannot on it be applied between one rib and another 

 as during life, and because the resistance to expansion is no longer of the same 

 description. On the prepared skeleton, by raising and depressing the sternum the 

 ribs may be moved upwards and downwards, parallel to one another ; the first rib 

 moving as freely as the others. But during life several causes combine to make the 

 first rib more fixed than those which follow : as for example, the weight of the upper 

 extremity, and the strain of the intercostal muscles and ribs beneath. The move- 

 ments of the thoracic walls in respiration are these. 1st. The antero-posterior dia- 

 meter is increased by a forward movement of the sternum ; the lower end of that 

 bone is raised and advanced, while the upper end, which in easy respiration is at rest, 

 or nearly so, is only raised in full inspiration. 2nd. The lateral diameter of the 

 thorax is increased, both by the elevation and the rotation of the ribs ; the first of 

 these movements bringing larger costal arches to a level occupied in expiration by 



