100 THE OSTEOLOGY AND MYOLOGY 



the central tendon ; the plane going to the xiphoid in the thinnest, and sometimes almost 

 divided along the median line by a membranous interval. The lateral fibres terminate 

 about an inch from the margin of the thorax, by separated flattened fasciculi, one for each 

 floating rib, interdigitatiug with those of the transversalis abdominis. The digitations 

 sometimes only fairly blend just before reaching the central tendon ; but there is great in- 

 dividual difference in this respect, as well as regarding the general thickness and fleshiness 

 of the muscle. The two crura are of unequal lengths ; the right, besides being longest, is 

 also stoutest. It ends on the body of the third lumbar ; the left usually between the 

 second and third ; both are directly continuous below with the anterior vertebral ligament. 

 The anterior margins of the crura are tendinous ; behind, muscular fibres descend to the in- 

 sertion of each crus, and are attached to the vertebra just above. The anterior tendinous 

 border of each crus curves over about an inch above its final insertion, to become continu- 

 ous with its fellow of the opposite side, overarching and enclosing the aortic opening, the 

 margin of which is thus, as usual, entirely fibrous. The muscular fibres of the two crura de- 

 cussate just above the aortic opening, at a slight angle, and then divaricate to surround the 

 oesophageal aperture. The latter is a fissure, bounded on both sides and below by muscle ; 

 above, it just reaches the border of the central tendon. Its left margin, representing fibres 

 that come from the right crus, is thicker than the other, and forms a well marked ridge or 

 fold. The foramen for the vena cava is situated above and to the right of the oesophageal, in 

 the central tendon, at the jvmction of the central and dextral leaflets. It is circular, and the 

 smallest of the three principal openings ; the fibres that form its periphery are thickened 

 and strengthened ; the most noticeable of these increased fibres is a set that proceeds from 

 the margin of the opening obliquely to the oesophageal fissure. The " ligamentum arcua- 

 tum " passes from the posterior margin of each crus to the transverse process of the first 

 lumbar, and base of the last rib, overarching the spine transversely. It is more or less 

 directly continuous with the fascia of the transvei'salis abdominis. The psoas parvus and 

 lumbar muscles pass up a little way behind it. I do not notice any " internal arcuate liga- 

 ment " well distinguished from the rest of this border of the muscle. 



Muscles of the Back. 



Of the numerous muscles of the back, those constituting the firet three "layers," as 

 usually described in anthropotomy, and part of those constituting the fourth and fifth 

 layers, are described in other connections, to wit : the trapezius, latissimus, rhomboideus 

 and levator anguli scapulae, with the muscles of the anterior extremity ; the two serrati 

 postici with the muscles of the thorax ; the sjjlenius, biventer, complexus, trachelo-mastoi- 

 deus, recti capitis superior and inferior, with the muscles of the posterior vertebral region 

 of the neck. The general disposition of the muscles that remain to be noticed is much the 

 same as in man ; the chief differences are found in the lumbar and sacral region, and relate 

 to the development of large caudal muscles. The following description may suffice to 

 exhibit the characters of the remaining muscles of the fourth and fifth layers, without 

 going into tedious and unnecessary detail. 



In the loins, that is, from the apex of the ilium to the last rib, there is on either side a 

 large sub-cylindrical mass of muscle, divisible into five parallel, longitudinal bundles. 



