284 THE ANATOMY OF THE HORSE. 



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 keep the anus closed except during 

 the passage of excreta. 



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

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

 present) from the superior ischiatic spine, and from the inner surface 

 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 muscle is to carry the anus forwards 

 and invert it after the act of defalcation. 



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

 rectum, 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 are then 

 prolonged downwards on the middle line of the corpus spongiosum, 

 on which they are lost near the extremity of the penis. 



Action. — To retract 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 vertebras. It forms at 

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



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

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

 and it passes transversely inwards to terminate on the middle line over 

 the urethra, being confounded with its fellow of the opposite side, and 

 with the first fibres of the accelerator urinae. 



