284 THE ANATOMY OF THE HORSE. 



arteries assume a coiled disposition, from which they receive the name 

 artence helicince. 



Structure of the corpus spongiosum. The structure of the spongy 

 body resembles, somewhat, that just described. It possesses an 

 envelope of fibrous tissue with trabecula3 and a plexus of large veins. 

 In its peripheral part, and in the bulb, it also contains true cavernous 

 spaces, like those of the cavernous bodies but smaller. 



Structure of the spongy (or extra-pelvic) part of the urethra. This 

 should be laid open on its under aspect with scissors. The lumen of 

 the tube is not uniform. At the ischial arch (this will not be seen at 

 present) it presents a dilatation ; and its calibre is again increased as it 

 enters the glans, forming what is termed in man the fossa navicularis. 

 The interior of the tube is lined by mucous membrane having simple 

 columnar epithelium, except at its orifice, where it is stratified and 

 squamous. The ducts of numerous small racemose glands open on the 

 surface of the membrane. External to the mucous membrane the wall 

 of the urethra is made up of non-striped muscular tissue, an^anged as an 

 inner circular and an outer longitudinal layer. 



