GLANDULAR EPITHELL\ 



15 



groups, and on the lips certain large follicles are set apart 

 for the production of the greatly enlarged sensory hairs or vibrissae. 

 Connected with the hair follicles are thin strands of smooth muscle, 

 the arrectores pilorum (Fig. 2, a, a'). They are placed in the 

 broad angles formed by the inclined follicles with the corium and 

 their contraction brings the hair into a more nearly erect position. 

 Important modifications are displayed in glandular epithelia and 

 sensory epithelia. 



Glandular Epithelia 



Epithelial glands are composed of epithelial cells which have 

 become greatly modified as secreting structures. In some cases the 

 secreting element is a single cell, a unicellular gland lying directly 

 in the general layer of epithelium. The mucus-secreting goblet cells 

 of the intestinal wall are structures of this nature. In other cases 

 groups of secreting cells form ingrowths 

 from the main layer, multi-cellular 

 glands. The lumen or cavity of the 

 gland, in most cases greatly complicated 

 through the division of the gland sub- 

 stance, is connected with the general 

 surface by a duct which serves to carry 

 away its secretion. In some cases, 

 however, the connection of a gland with 

 the epithelial surface is only embryonic 

 and, in the adult condition, the gland is 

 found separated from the epithelium 

 from which it was originally formed. 

 This condition is represented by the 

 thyreoid and thymus glands of the neck 

 and thorax respectively. Typical epi- 

 thelial glands are accordingly external 

 secreting, or exocrine glands, their se- 

 cretions being discharged through ducts 

 to free surfaces; while those which lose 

 their ducts are internal secreting, or 

 endocrine glands, their products being absorbed directly into 

 the blood or lymph (cf. p. 131). There are many structures, 

 such as the suprarenal gland and part of the pituitary body, 



Fig. 3. Diagrams of main types 

 of multicellular glands. A, simple 

 tubular gland; B, coiled tubular 

 gland; C, branched tubular gland; 

 D, simple acinous gland; E, 

 branched or compound acinous 

 gland. 



