298 THE ANATOMY OF THE HORSE. 



Muscles of the Abdominal Wall. On each side there are four of these, 

 viz., the obliquus abdominis externus, the obliquus abdominis interims, 

 the rectus abdominis, and the trans versalis abdominis. They are stated 

 in the order of their occurrence, the first being the most external. 

 These muscles have not only to discharge the ordinary function of a 

 muscle, but they have also to close in the abdominal cavity ; and, for 

 this latter purpose, they are, with the exception of the rectus abdominis, 

 peculiarly modified in form. Thus, the two oblique muscles and the 

 transverse muscle have their tendons of insertion extended in the form 

 of great fibrous or aponeurotic sheets, and the fibres in each of these 

 tendons have a direction different from that of the others. 



The Linea Alba is the white mesial raphe, or band, which extends 

 from the ensiform cartilage to the pubes. It is fibrous in structure, and 

 is formed by the meeting of the aponeurotic tendons of the right and 

 left muscles. A little behind its mid point is a puckered cicatrix — the 

 umbilicus. 



The External Abdominal Ring (Plate 39). This is the lower orifice 

 of the inguinal canal. It has the form of a slit in the tendon of the 

 external oblique. The direction of the slit is oblique forwards and out- 

 wards. The lips, or pillars, of the slit are simply fibres of the external 

 oblique tendon. The inner angle or commissure is placed at the edge of 

 the prepubic tendon. This prepubic tendon is a strong fibrous band by 

 which the abdominal muscles get a common insertion into the anterior 

 edge of the pubic bones, and from whose surface the pubio-femoral liga- 

 ment of the hip-joint arises. The external abdominal ring gives passage 

 in the male to the spermatic cord, the external pudic vessels, and the 

 inguinal nerves. In the female it transmits merely the corresponding 

 vessels and nerves. 



The Obliquus Abdominis Externus (Plate 39). This consists of a 

 muscular part at its antero-superior edge, and an aponeurotic tendon 

 over the inferior and lateral parts of the abdomen. It arises by its 

 muscular portion from the outer surface of the last fourteen ribs, and 

 behind the last rib from the tendon of the latissimus dorsi. Its anterior 

 slips of origin interdigitate with the serratus magnus. The muscular 

 fibres are directed obliquely downwards and backwards, and are succeeded 

 by the aponeurotic tendon. The fibres of the tendon continue in the 

 same direction, and become inserted into the linea alba, the prepubic 

 tendon, and the external angle of the ilium ; while between the two last- 

 mentioned points they are continued to form Poupart's ligament. Along 

 the line between these two points the fascia of the inside of the thigh is 

 inserted to the surface of the tendon, and it must be cut in order to 

 expose the ligament. It will then be observed that from the prepubic 

 tendon to the bony prominence of the haunch the fibres of the external 

 oblique tendon, instead of becoming inserted into bone, curve upwards 



