558 



HUMAN ANATOMY. 



The dome does not, however, form a simple curve, but is divided by a median 

 depression, which traverses it from before backward, into two secondary lateral 

 domes which are unequally developed, that of the right side extending upward as 

 far as the level of the junction of the fourth costal cartilage and rib, while that of 

 the left reaches only to the fifth costo-cartilaginous junction. 



Nerve-Supply. — From the third, fourth, and sometimes the fifth cervical 

 nerves, by the phrenic nerves. 



Action.. — To increase the vertical diameter of the thorax, a contraction of the 

 muscle-fibres depressing the summit of the dome. 



Relations. — The upper surface of the diaphragm forms the floor of the thoracic 

 cavity and is in contact with the pleurae and pericardium, the latter being adherent to 

 the centrum tendineum. Below, the diaphragm is largely invested by peritoneum, and 

 is in relation with the liver, stomach, spleen, kidneys, suprarenal bodies, duodenum, 

 pancreas, inferior vena cava, and the branches of the cceliac artery. 



Variations. — Occasionally the diaphragm is incomplete in its posterior portion, a condition 

 which permits the formation of congenital diaphragmatic hernias. Embryologically the pos- 

 terior portion of the diaphragm is the last to form, and in this fact is probably to be found an 

 explanation of the location of this imperfection and also of the course of the phrenic nerves 

 anterior to the roots of the lungs to reach the earlier formed anterior portion of the diaphragm. 



Fibres which arise from the crura and pass to neighboring structures are frequently present. 

 Among the more constant of these are fibres which arise from the inner borders of both crura 

 and pass to the lower portion of the oesophagus, mingled with dense connective-tissue fibres, 

 and others which pass from one crus or the other into the mesentery of the upper part of the 

 jejunum. Probably the suspensory musc/e of the duodenum, or muscle of Treitz, which passes 

 from the left crus to the terminal portion of the duodenum, belongs to tTiis latter group of fibres, 

 although it has been stated to be formed by non-striated muscle-fibres. 



THE PELVIC AND PERINEAL MUSCLES. 



The ventral portions of the myotomes succeeding the first lumbar and from that 

 as far down as the third (or second) sacral are almost entirely unrepresented in the 

 trunk, being devoted to the formation of the musculature of the lower limb. Below 



Fig. 550. 



Iliac crest 



Iliac fascia 



ternal iliac vessels 



Ilio-pectineal line 

 Acetabulum 

 Levator ani 



Obturator intern us -^iy!^ : 

 /,»;;- bi ' , 



Alcock's canal _:'i;:£LYl-J 



vie fascia 

 Obturator fascia 

 Anal fascia 



Vina 



Ischio-rectal fossa 



Seminal vesi( 

 External 



Internal sphincter Rectum 



Diagrammatic frontal section through pelvis, showing relations of fascial layers to pelvic wall and floor. 



the point mentioned, however, the ventral musculature again appears in the trunk 

 in the pelvic, the perineal, and occasionally the coccygeal region. Owing to the 

 conditions under which it appears, it is not possible to refer the muscles derived from 

 it to the various subdivisions into which the ventral musculature of other regions is 

 divisible, and they will therefore be considered in sequence without any attempt at 

 classification other than regional. 



The Pelvic Fascia. — The pelvic fascia is attached above to the promontory 

 of the sacrum and the ilio-pectineal line (linea terniinalis) of the pelvis, where it 



