THE DIAPHRAGM 



437 



to the tendon the diaphragm projects higher into the thoracic cavity than in the central area. 

 On the right, in moderate expiration, it extends in adults to the height of the medial extremity 

 of the fifth rib, and on the left to the fifth interspace. 



Origi7i.— On each side from — (1) the lower border and back of the xiphoid process and the 

 adjacent aponeurosis of the transversus abdominis or from the tendinous arch extending from 

 the tip of the xiphoid process to the cartilages of the fifth and sixth ribs, {sternal portion); (2) 

 the lower border and inner surfaces of the cartilages of the seventh and eighth ribs, the cartilages 

 and osseous extremity of the ninth rib and the osseous extremities of the last three ribs {costal 

 portion); and (3) from the lumbar vertebrae {lumbar portion). The lumbar portion is divided 

 somewhat irregularly into three crura, between which pass blood-vessels and nerves. 



The lateral crus arises from the lateral surface of the bodies of the first two lumbar vertebrae 

 and from fibrous thickenings of the fascia over the psoas and quadratus lumborum muscles. 

 Of these, one, the medial lumbo-costal arch (internal arcuate hgament), extends from the body 

 of the second lumbar vertebra to the transverse process of the same vertebra; the other, the 

 lateral lumbo-costal arch (externa.1 arcuate hgament ). extends from the tip of the transverse 



Fig. 392. — The Perineal Muscles in the Female. 

 Pubis 



Clitoris 



Vagina 



Superficial layer of 

 urogenital trigone 



Tuber ischii 



Sphincter ani exter- 

 nus profundus 



Pubo-coccygeus 



Ilio-coccygeus 

 Sphincter ani exter- 



nus subcutaneus 



Sphincter ani exter- 



nus superficialis 



Constrictor radicis 

 clitoridis 



-Ischio-cavernosus 



Bulbo-cavernosus 



Transversus 

 perinei 



Obturator internus 



Coccygeus 

 Gluteus maximus 



Coccyx 



process of the second lumbar vertebra to the_tj\-elfthxibj, The lateral crus is only inconstantly 

 attached to this. The intermediate crus arises frOm the ventro-lateral surface of the body of 

 the second lumbar vertebra from the sides of the bodies of the first two lumbar vertebrae and 

 from the intervening discs. The medial crus arises from the front of the bodies of the third and 

 the fourth lumbar vertebrae. On the left side it usually extends only to the third vertebra, 

 and it does not always extend to the fourth on the right. The extremity and medial margin 

 of this crus are tendinous, the lateral portion fleshy. On the second, third, and fourth, and the 

 lower part of the first lumbar vertebrae the medial crus of each side is separated from its fellow 

 by the hiatus aorticus (for the aorta and thoracic duct). Over the first lumbar vertebra they 

 are fused by a process which extends from the right crus into the lower ventral surface of the 

 left. Above here the right crus may be divided into two parts, one of which, fused with the 

 left crus, passes on the left of the hiatus oesophageus, while the other passes on the right. Some- 

 times the hiatus oesophageus Ues between the right and left crura. Frequently the left crus gives 

 off a slip which passes to the ventral surface of the right below the hiatus. 



The costal portion arises by a series of dentations which do not correspond perfectly in 

 number with the ribs. Some costal cartilages have two dentations attached to them. It 

 interdigitates with the transversus abdominis but in part arises from tendinous arches which 

 bridge the origin of the transversus in the last three interspaces. 



Structure and insertion. — The central tendon has somewhat the shape of a trifoliate leaf, 



