358 THE ANATOMY OF THE HORSE. 



it two muscles. The first of these, termed Wilson's muscle, or the 

 constrictor urethral, envelops the tube behind the prostate gland, from 

 which, indeed, it is not well defined. Its muscular fibres, of a pale red 

 colour, comprise two sets, w T hich extend across the urethra on its upper 

 and lower faces respectively, and embrace the tube like an elliptical 

 sphincter. The most posterior fibres of the muscle pass over Cowper's 

 glands. The other, termed the ischio-urethral muscle, consists on each 

 side of a band whose fibres arise from the ischial arch, and pass to the 

 urethra beneath Cowper's gland, blending with Wilson's muscle. Like 

 the preceding, it is composed of pale red muscular fibres. 



Action. — These muscles are constrictors of the membranous urethra, 

 and aid in the ejection of urine and semen. 



The spongy portion of the urethra, with its muscles — the transversus 

 perinsei and accelerator urinse — has been already described as a con- 

 stituent part of the penis (page 292). 



The Prostate Gland (Plates 46 and 47) embraces the neck of the 

 bladder and the initial part of the urethra. It consists of a middle and 

 two lateral lobes ; and in structure it is glandular, with a considerable 

 admixture of striped muscular tissue. Its glandular texture consists of 

 branching excretory tubes and acini, both having a columnar lining. 

 Its ducts, as will be seen at a later stage, perforate the urethral wall, to 

 which it is adherent. 



Cowper's Glands (Plates 46 and 47). Each of these is placed at the 

 side of the membranous urethra, just in front of the ischial arch. They 

 are round, reddish-yellow, and (in the stallion) about the size of a hazel 

 nut. They have the racemose type of structure, and their ducts 

 perforate the adjacent wall of the urethra. 



The Vasa Deferentia (Plates 46 and 47). These are the excretory 

 ducts of the testicles. As already seen, each is one of the constituents 

 of the spermatic cord. Appearing at the internal abdominal ring, as a 

 tube about the thickness of a goose-quill, it is reflected backwards to 

 enter the pelvis. Crossing the direction of the ureter, it places itself on 

 the upper surface of the bladder, and expands to four or five times its 

 previous calibre, forming what is called the bulbous portion of the vas 

 deferens. It then passes backwards beneath the vesicula seminalis ; 

 and, contracting again, it terminates under the prostate, by uniting out- 

 wardly with the neck of the vesicula to form a short tube termed the 

 ejaculatory duct. Where the vasa deferentia lie above the bladder they 

 are connected together by a peritoneal fold, Douglass's fold, between 

 whose layers there is contained the vesicula prostatica, or uterus mascu- 

 linus. This is a short tube with a blind anterior end, and opening by 

 its posterior extremity into the urethra. It is the homologue of the 

 uterus and vagina of the female. 



The Vesicula Seminales (Plates 46 and 47). These bodies are 



