i8 



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 (E) 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. 



FiG. ii. Different types of glands; .4, to D, tubular; E, F, acinous; A, simple; B, coiled; 



CF, 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, ). 



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 by a 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. 



