300 THE ANATOMY OF THE HORSE. 



penis. In the mare the latter branch is represented by the mammary 

 artery. 



The External Pudic Vein is proportionally smaller than the artery, 

 which it accompanies. 



The Inguinal Nerves are derived from the 2nd and 3rd lumbar 

 nerves, and are distributed to the prepuce, the scrotum, and the 

 adjacent skin. 



Directions. — Incise the external oblique tendon, from the external 

 angle of the ilium to the edge of the prepubic tendon. Reflect Poupart's 

 ligament towards the thigh, and hook it up after the manner of Plate 40. 

 Then strip away the tendon of the external oblique from the subjacent 

 internal oblique. This will be found easy in the region of the flank, 

 where the tendon is related to the muscular part of the internal oblique ; 

 but over the inferior part of the abdomen, and especially in front, where 

 the tendons of the two muscles are applied to each other, the operation 

 is difficult, and in some parts impossible. In this proceeding the 

 dissector has to guard against removing the thin tendon of the inner 

 muscle along with the outer, and this he will best do by observing that 

 the fibres of the inner tendon cross these of the outer at right angles, 

 being directed downwards and forwards. Observe that anteriorly the 

 two tendons are not simply in apposition, but actually interwoven — a 

 disposition of tendons which is unique, and one which greatly increases 

 the strength of the abdominal floor. The muscular portion of the 

 external oblique should be raised as far as the lower extremities of the 

 ribs. A better view of the inguinal canal and its contents will now be 

 obtained. 



The Obliquus Abdominis Internus (Plate 50) consists of a fan-shaped 

 fleshy portion situated in the flank, and an aponeurotic tendon spread 

 over the abdominal floor. It arises from the external angle of the ilium, 

 and from the adjacent part of Poupart's ligament. It is inserted into 

 the prepubic tendon and the linea alba by the inferior edge of its tendon, 

 and by tendinous slips into the four or five last costal cartilages. In 

 front of the lower end of the fourth last intercostal space, the aponeurotic 

 tendon has a free edge which ordinarily lies under concealment of the 

 line of overlapping costal cartilages. When the abdomen is tympanitic, 

 however, this edge is thrust outwards, and the transversalis muscle is 

 exposed as in Plate 40. The posterior edge of the fan-like muscular 

 portion lies in contact with Poupart's ligament ; and the inguinal canal, 

 as already seen, passes between the two structures. The highest fibres 

 of its muscular part are parallel to the edge of a small muscle — the 

 retractor costos — inserted into the last rib, under cover of the most 

 posterior slip of the serratus posticus. This is described with the 

 muscles of the back (page 98). 



Action. — Similar to that of the external oblique. 



