THORAX AND ABDOMEN OF THE HORSE 87 



as yet hidden from view, the artery divides into two branches — cranial 

 and caudal. The cranial branch lies between the transverse and 

 internal oblique muscles and divides into two rami. One of these 

 pursues a horizontal course towards the last rib ; the other slopes 

 obliquely in a cranial and ventral direction. The caudal branch pierces 

 the abdominal wall close to the tuber coxse, follows the medial surface 

 of the tensor muscle of the fascia lata towards the fold of skin that 

 connects the wall of the abdomen to the distal part of the thigh. A 

 group of lymph glands (lymphoglanduiae subiliacse) is associated with 

 this branch. 



M. TRANSVERSUS ABDOMINIS. — The transverse is the weakest 

 member of the group of muscles entering into the formation of the 

 abdominal wall. Part of its muscular fibres (pars costalis) arise from 

 the cartilages of the last twelve or thirteen ribs., where they form 

 serrations interdigitating with similar serrations of the diaphragm. 

 The rest of the origin (pars lumbalis) is from the ends of the transverse 

 processes of the lumbar vertebrae. The fibres of the muscle follow a 

 course at right angles to the long axis of the body, and soon give place 

 to a relatively thin aponeurosis. This passes underneath the straight 

 muscle, of which it forms the dorsal sheath, and blends with the linea 

 alba. 



The deep surface of the transversus abdominis muscle is covered by 

 a thin layer of fascia — fascia transversalis — in which, in well-nourished 

 subjects, there may be considerable deposit of fat. The fascia trans- 

 versalis is continuous with the iliac fascia, and is prolonged down the 

 inguinal canal to form the fascial tunic lined by serous membrane that 

 constitutes the innermost layer of the scrotum. 



The sheath of the rectus abdominis muscle (Vagina m. recti 

 abdominis). — The sheath in which the straight muscle is enclosed has 

 now been completely revealed. Its superficial or ventral layer is the 

 stronger, and is formed by the interlacing fibres of the aponeurotic 

 tendons of the two oblique muscles. The deep or dorsal layer of the 

 sheath, formed by the transverse muscle aponeurosis, is thinner, and 

 particularly so close to the pubis. Where the straight muscle lies over 

 the costal cartilages no sheath is interposed. 



Where the sheaths of the two straight muscles meet in the middle 

 line a strong cord is formed. This is the linea alha, which stretches 

 from the xiphoid process of the sternum to the symphysis pubis, where 

 it blends with the strong conjoined tendons of the two recti muscles. 

 Composed mainly of the interlacing fibres of the oblique and transverse 

 tendons, but also containing a few longitudinal fibres, it is strongest 



