206 



THE ESSENTIALS OF HISTOLOGY. 



tissue ; within this an equally thick layer of circular bundles of the same 

 tissue, and within this again a thin layer of longitudinal muscle. The 

 tube is lined by a mucous membrane, the inner surface of which is 

 covered by columnar non-ciliated epithelium. 



The ampullce of the vasa deferentia, and the vesiculce seminales, are 

 in structure similar to the vas deferens, but their corrugated walls 

 are much thinner. 



The connective tissue between the tubules of the testis is of very 

 loose texture, and contains numerous lymphatic clefts, which form an 

 intercommunicating system of commencing lymphatic vessels. Lying 

 in this intertubular tissue are strands of polyhedral epithelium-like 

 cells (interstitial cells, see fig. 238) of a yellowish colour; they are 

 much more abundant in some species of animals (cat, boar) than in 

 others. They accompany the blood-vessels before these break up to 

 form the capillary networks which cover the walls of the seminiferous 

 tubules. 



The spermatozoa. Each spermatozoon consists of three parts, a 

 Jiead, a middle part or body, and a long tapering and vibratile tail. 

 In man (fig. 240) the head is of a flattened oval shape, 

 somewhat more flattened and pointed anteriorly ; it is 

 said to be provided with a small barb-like projection 

 at its extremity (Dowdeswell). The middle-piece is 

 short and cylindrical, and appears to have a spiral 

 fibre passing round it. The tail is the longest part of 

 the spermatozoon, and during life is in continual 

 vibratile motion, the action resembling that of the 

 cilia of a ciliated epithelium-cell. The extremity of 

 the tail (end-piece) forms a distinct part of the 

 spermatozoon, and in some animals may become 

 split up into two or three fibrils. Human sperma- 

 tozoa are about ^J^-inch long. In different animals 

 the shape of the head and the extent of the middle- 

 piece and tail vary greatly. In the rat (fig. 242, 7) 

 the head is long, and is recurved anteriorly; it is 

 set obliquely on the middle -piece, which is also of 

 considerable extent, and has a closely wound spiral 

 filament encircling it in its whole length (H. H. 

 Brown). In the newt the head is long and tapering, 

 and the tail appears to have a membranous expansion, 

 attached in a spiral manner along its whole length. This has also 

 been described in the human spermatozoon, but its existence here is 

 doubtful. 



FIG. 240. HUMAN 

 SPERMATOZOA. 

 (Retzius.) i- /^. 



1, in profile; 2,viewed 

 on the flat; b, head; 

 c, middle-piece ; d, 

 tail ; e, end-piece of 

 the tail, which is de- 

 scribed as a distinct 

 part by Retzius. 



