THORAX AND ABDOMEN OF THE HORSE 71 



nected with the superficial opening of the inguinal canal. Along the 

 lateral wall of the sac there is a flattened band of red muscular tissue, 

 the exter7ial cremaster ^ muscle (m. creniaster externus), which a later 

 dissection will show to be associated with the internal oblique muscle 

 of the abdominal wall. Where the cremaster appears at the sub- 

 cutaneous ring of the inguinal canal, it is narrow. From this point its 

 fibres diverge somewhat and end in an aponeurosis blended with the 

 fibrous sac upon which it lies. 



The innermost and thinnest stratum of the scrotal wall is formed by 

 the serous layer of the vaginal tunic, which is continuous through the 

 inguinal canal with the peritoneal lining of the abdomen, and, like 

 other serous membranes, is divisible into a parietal and a visceral part. 

 The potential space between the two parts is known as the vaginal 

 cavity (cavum vaginale), and has a comparatively narrow connection 

 with the peritoneal cavity through the vaginal ring (annulus vaginalis). 

 The parietal part of the serous layer of the tunica vaginalis lines and is 

 closely adherent to the fibrous layer, and forms a smooth and glistening- 

 lining to the scrotal sac. The visceral portion, which will be examined 

 shortly, forms a smooth, moist and shining covering to the testis and 

 certain associated structures. 



Dissection. — Cut through the parietal tunica vaginalis with a pair of 

 scissors and examine the contents of the scrotum. 



Testis and epididymis. — Each testis, weighing from 230 to 

 300 grammes, is an ovoid organ, slightly flattened medially, lodged 

 within its own compartment of the scrotum. The long axis of the 

 gonad is approximately horizontal ; consequently its extremities are 

 cranial and caudal. The two surfaces, medial and lateral, are smooth 

 and convex, as is also the ventral or free border (margo liber). The 

 dorsal border (margo epididymidis), comparatively straight, is associated 

 with the epididymis, and affords attachment to the spermatic cord. 



The epididymis^ consists of an elongated mass formed by the 

 tortuous windings of a long tube — the duct of the epididymis (ductus 

 epididymidis) — held together by dense connective tissue, and covered 

 by visceral tunica vaginalis. The connective tissue covering of the 

 surface is intruded into the interior of the epididymis in the form of 

 strong septa that divide the organ into lobules (lobuli epididymidis). 



The central part or body of the epididymis (corpus epididymidis) 

 lies along the dorsal border of the testis to which it is attached by the 

 vaginal tunic. The cranial and caudal extremities, known respectively 

 as the head (caput epididymidis) and tail (cauda epididymidis), overlap 



^ Cremaster [L.] ; Kpefiacrrrip (cremaster) [Gr.], a suspender. 

 2 eirl (epi) [Gr.], upon ; Sldvfios (didymos) [Gr.j, double, twain, testis. 



