86 GLANDS. 



glands, which will then serve as types, around which 

 those of similar structure will naturally group them- 

 selves. And let us commence by examining those of 

 simplest structure, e. g. the salivary glands, 

 salivary glands, /Salivary Glands. On placing a very thin section 

 of the sub-lingual gland under the microscope, we see 

 that its terminal cul-de-sacs, or ccecal pouches, are 

 incomplete vesicles, the walls of which are formed by 

 two layers. The external layer is basement mem- 

 brane, structureless and exceedingly thin, being only 

 TeVoth of a line in thickness. Its inner lining con- 

 sists of a layer of many-sided cells, with extremely 

 pale outlines, and averaging in diameter aiirth of a 

 line. Their nuclei are much more distinct, and so 

 large as to almost fill the cells (PL XVIII. fig. I. 1). 

 Three or four of these vesicles, connected -together 

 closely in a group, have an outlet in common, and 

 constitute thus a microscopic lobule. Several of these 

 minute excretory canals, each with its corresponding 

 lobule, meet together in a canal of somewhat larger 

 size, and by their union form a lobule large enough 

 to be seen by the naked eye (PL XVII. fig. V). 

 Finally, the aggregation of a number of lobules, with 

 a canal of proportionate size, results in the formation 

 of lobes, and these, in their turn, uniting in a common 

 excretory duct, make up the gland. The excretory 

 duct is composed externally of a coat of connecting 

 tissue, and internally of a layer of cylindrical epithe- 

 lium. The gland is enveloped .by a membranous 

 expansion of connecting tissue, from the internal sur- 

 face of which laminated processes are sent into the 

 substance of the organ, where they invest the exte- 



