MUSCLES OF THE TRUNK 307 



the muscular portion. Its internal border is fixed to the xiphoid cartilage and 

 the white line. Its posterior border, badly defined, appears to join the crural 

 arch only on its outer aspect. 



Relations. — Outwards, with the inferior extremity of the asternal ribs and 

 their cartilages, with the rectus abdominis, the small oblique, and the depressor 

 muscle of the last rib ; inwardly, with the peritoneum, from which it is separated 

 by the subperitoneal aponeurosis — an extremely fibrous layer which, in Man and 

 some animals, becomes much thickened towards the crural arch, where it forms 

 adhesions. It has been described, in human anatomy, as the fascia transversalis. 



Action. — It compresses the abdominal viscera when it contracts, and presses 

 them against the vertebral column. 



Differential Characters in the Muscles of the Abdominal Kegion in the otheb 



Animals. 



A. Ruminants. — The development of the tunica ahdominalis is in proportion to the 

 volume of the digestive viscera. This membrane is, therefore, very wide and thick in Rumi- 

 nants. 



The obliquus externus has no femoral aponeurosis; its aponeurosis is therefore entirely 

 reflected in the abdominal cavity. The inguinal canal is very short ; its upper orifice is very 

 narrow, and it is situated near where the two portions of the long adductor of the leg unite 

 (Goubaux); its inferior opening is very elongated from before to behind, and without to 

 within. 



The muscular portion of the small oblique occupies the entire space comprised between the 

 posterior border of the last rib, the extremity of the transverse processes of the lumbar vertebrae, 

 and the external angle of the ilium. The small retractor of the last rib is not distinct from the 

 principal muscle. The rectus abdominis is wide behind, with tendinous intersections more 

 marked at its superior than its inferior face ; the aponeurosis of the transversalis is much 

 thicker and more resisting than in Solipeds. 



The prepubic tendon of the abdominal muscles is large, and attached to the inner face of 

 the short adductors of the leg by two ligamentous bands ; this attachment depresses the lower 

 part of the abdominal wall, and therefore it is that, in Ruminants, there is a concave depression 

 in front of the pubis. 



B. Pig and Camivora. — The tunica ahdominalis is reduced to an insignificant layer. 

 The external oblique is remarkable for the enormous development of its muscular portion and 

 the narrowness of its aponeurosis. The internal oblique resembles that of Ruminants. 



Comparison of the Abdominal Muscles of Man with those of Animals. 



With the exception of some sligbt diff'erences, the abdominal muscles of Man resemble 

 those of the smaller animals. The tunica ahdominalis does not exist, but is represented by a 

 layer of connective tissue which separates the skin from the aponeurosis of the obliquus 

 externus. The muscular portion of this muscle has no attachment to the aponeurosis of the 

 great dorsal. 



The aponeurosis of the obliquus internus is divided into two layers at the external border of 

 the rectus muscle; the anterior is consolidated with the external oblique, and passes in front 

 of the rectus; the posterior is united to the transversalis, and passes behind that muscle. 



The aponeurosis of the transversalis is divitied into two layers, only one of which femains 

 behind the rectus; this is named the semilunar fold of Douglas. 



The rectus offers three transverse fibrous intersections in its length (linem transversse). At 

 its upper extremity, it divides into three branches: the internal is attached to the xiphoid 

 appendage and the cartilage of the seventh rib; the middle, to that of the sixth rib; the 

 external, to the cartilage of the fifth rib. 



" To the rectus is annexed a small triangular muscle, the pyramidalis, which is not 

 found in animals. This muscle is about 2^ inches long; is sometimes absent; most developed 

 in children ; is attached by its base to the pubis, between the spine and the symphysis ; and 

 by its summit is continuous with a tendon which is lost in the white line, and constitutes, 

 with that of the opposite siile, a fibrous cnrd whicii may be followed to the umbilicus"* 

 (Beaunis and Bouchard). 



Lastly, at the inner aspect of all the abdominal muscles, beneath the peritoneum, is a fibrous 



