1 66 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



two domes of the diaphragm, and two leaflets which are directed posteriorly. The left leaflet is 

 the smaller and forms the left dome of the diaphragm; the right is larger and forms the right 

 dome. At the base of the right leaflet near its posterior margin is situated a large irregular 

 rounded opening, completely within the central tendon, which gives passage to the inferior vena 

 cava and is designated the opening for the vena cava (quadrilateral foramen). 



The curvature of the diaphragm is not uniform, but there is a middle lower portion and two 

 lateral domes which project markedly toward the thoracic cavity. The right dome is more capa- 

 cious and extends to a higher level than the left; its highest point corresponds to the fourth, that 

 of the left to the fifth intercostal space. Posteriorly the diaphragm (the lumbar portion) extends 

 much lower than it does anteriorly. Its transverse is considerably larger than its sagittal 

 diameter. 



The diaphragm possesses a series of foramina and spaces which give passage to vessels or 

 nerves. These are: (i) The aortic opening, which is only partly formed by the diaphragm; (2) 

 the esophageal opening, purely muscular and formed entirely by the diaphragm ; (3) the opening 

 for the vena cava, situated entirely within the tendinous portion of the muscle; and (4) the slit-like 

 spaces between the inner and middle crura and between the middle and external crura. The 

 latter spaces give passage to the vena azygos, to the vena hemiazygos, and to the sympa- 

 thetic and the splanchnic nerves, which are arranged in a variable manner. In addition to the 

 aorta, the aortic opening also transmits the thoracic duct. 



The motor nerve of the diaphragm is the phrenic nerve from the cervical plexus. 



The diaphragm is the chief muscle of respiration. By the contraction of its fibers the domes of the diaphragm 

 are drawn downward and the costal portions are drawn away from contact with the thoracic wall, so that the thoracic 

 cavity is increased in size and the abdominal cavity is diminished. 



[The diaphragm, from the developmental standpoint, belongs to the cervical musculature, the muscular tissue which 

 it contains being derived from the fourth (and to a certain extent from the third and fifth) cervical myotomes; the entire 

 structure lying at one period of the development in the cervical region and later migrating downward to its final posi- 

 tion between the thorax and abdomen. Hence it is that it is supplied by the phrenic nerve, which arises from the fourth 

 (third to fifth) cervical nerve, and elongates in proportion as the diaphragm recedes toward its final position. — Ed.] 



THE THORACIC MUSCLES. 



The muscles of the thorax (Figs. 245, 247, 253, and 254) are composed of two main groups: 

 (1) Those which arise from the thoracic skeleton and insert into the skeleton of the upper extrem- 

 ity; these are, consequently, really muscles of the extremity; and (2) the actual muscles of the 

 thoracic wall, which are known as the intercostales. 



The first group is arranged in three layers which are not exactly superimposed. The first 

 layer is formed by the pectoralis major, the second by the pectoralis minor and the subclavius, and 

 the third by the serratus anterior. 



THE THORACIC MUSCLES OF THE UPPER EXTREMITY. 

 The First Layer. The Pectoralis Major. 

 The pectoralis major (Fig. 245) is a large, flat, thick muscle which is situated in the sternal, 

 infraclavicular, mammary, axillary, and inframammary regions, its outer border forming the 



