276 THE ANATOMY OF THE HORSE. 



In this part of its course the vessel gives off small branches to the 

 anus and to the erector penis muscle. 



The Internal Pudic Vein accompanies the artery. 



The Sphincter Ani Externus. The fibres of this muscle are of the 

 striped variety, and they are circularly disposed around the anus. 

 Above the anus the fibres are fixed at the root of the tail, and below it 

 they unite to form a pointed slip inserted into the perineal fascia. The 

 muscle should be removed in order to expose the next. 



The Sphincter Ani Internus. This is comprised between the outer 

 muscle and the mucous membrane. Its fibres are circularly disposed 

 like those of the external sphincter, from which they differ in being of 

 the non-striped variety. They are, in fact, nothing more than the last 

 of the circular muscular fibres of the rectum ; and in the horse they are 

 not aggregated in the form of a ring, as they are in man. 



Action of the sphincters. — To maintain the anus closed except during 

 the passage of excreta. 



The Retractor Ani (Levator ani of human anatomy). This nmscle is 

 red like the external sphincter. It arises (but this cannot be seen at 

 present) from the superior ischiatic spine, and from the inner sm-face of 

 the sacro-sciatic ligament over the small sacro-sciatic foramen. Its 

 fibres pass upwards and backwards, and terminate in tendinous slips 

 that are insinuated beneath the anterior edge of the external sphincter. 



Action. — During the passage of fasces the anus is carried backwards 

 and everted, and the action of this m\iscle is to carry the anus forwards 

 and invert it after the act of defsecation. 



The Retractor Penis. This muscle descends at the side of the rec- 

 tum, immediately in front of the external sphincter, and under cover of 

 the termination of the retractor ani, which must therefore be raised and 

 turned forwards. The fibres of the muscle are non-striped, and they 

 form a narrow riband which arises from the 1st and 2nd or 2nd and 

 3rd coccygeal bones. The right and left bands meet below the rectum, 

 for which they thus form a kind of sling. They arc then prolonged 

 downwards on the middle line of the corpus spongiosum, on which they 

 are lost near the extremity of the penis. 



Action. — To i-etract the penis within the prepuce when erection passes 

 off". 



The Suspensory Ligament of the Rectum (Plate 46). This, although 

 denominated a ligament, is composed of non-striped muscular tissue. It 

 is derived from the longitudinal muscular fibres of the rectum, which it 

 leaves in front of the external sphincter; and passing upwards, it 

 becomes inserted into the 4th and 5th coccygeal vertebrae. It forms 

 at the root of the tail a prominence which has already been referred to. 



The Transversus Perin^i. This muscle is not constantly present. 

 It arises from the tuber ischii, behind the origin of the erector penis ; 



