18 INTRODUCTION. 
exceeds their diameter; while in squamous epithelium, the cells are 
thin and flat, covering the largest amount of surface with the least 
amount of material (B). Sometimes the epithelial cells are in a single 
layer, forming simple epithelium (A, B, C);-in other places there are 
several layers—the epithelium is stratified (D, E, F). 
Frequently epithelia, usually of the columnar variety, are called upon 
to move fluids slowly; then the free surface is covered with minute 
vibratile hairs or cilia (EZ) which create currents. In glandular 
epithelium the cells, usually cubical or columnar, are specialized for 
the elaboration of secretions to be used by the animal or of waste prod- 
ucts (excretions) to be voided from the body. 
Fic. 11.—Different types of glands; A, to D, tubular; E, F, acinous; A, simple; B, coiled; 
Ce, branched. 
Glands.—The chief kinds of glands may be mentioned here. All have for 
their function the extraction and elaboration of certain products from the blood, 
consequently they have a good blood supply. Glands may be unicellular or multi- 
cellular according as they consist of isolated cells or of-many cells. In unicellular 
glands (abundant in the digestive tract) each cell passes its own secretion directly 
to the place where it is to be used (fig. 19, 2). 
Multicellular glands occur where a large amount of secretion is necessary in a 
limited space, hence they are not on the surface but at some deeper point, and their 
product is conveyed to the desired place bya duct. Multicellular glands are of two 
structural kinds. In the tubular gland the whole is approximately of the same 
diameter throughout, with little differentiation of gland and duct. It may be 
simple (A) or coiled (B) or branched (C, D), these modifications serving to in- 
crease the secreting surface. In acinous glands (D, E) there is a marked’ differ- 
ence between gland and duct, the glandular part forming an enlargement (acinus) 
on the end of the duct. Both simple and compound acinous glands are common. 
Still another type of gland, the ductless or ‘internal secretion’ gland occurs. 
In this there is no duct, the secretion elaborated by the cells passing by osmose into 
the blood-vessels. These secretions, collectively known as hormones, have 
recently acquired great prominence from their influence on different organs. 
