22 



Anatomy of the Rabbit. 



the general surface of the body the hair follicles are arranged in 

 groups (Fig. 5), 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 throws the hair into a more nearly erect position. 



Epithelial glands are ingrowths 

 A of the general layer, the cells of which 



become greatly modified as secreting 

 structures. The lumen or cavity 

 of the gland, in most cases greatly 

 complicated through the division of 

 the gland substance, is connected 

 with the general surface by a duct 

 which serves to carry away its 

 secretion. In some cases the con- 

 nection of a gland with the epithelial 

 surface is 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. In 

 other cases the secreting element is 

 a single cell, the latter thus repre- 

 senting a unicellular gland lying 

 directly in the general layer. The 

 mucus-secreting goblet cells of the 

 intestinal wall are structures of this nature. Typical epithelial 

 glands are accordingly external secreting glands in the sense that 

 the secretion is discharged and utilized on a free surface. There 

 are many structures, such as the suprarenal and pituitary bodies, 

 commonly described as internal secreting glands, the products of 

 which as hormones or chemical bodies of various kinds have a 

 modifying effect upon functions in outlying parts of the body. 

 They are not of necessity epithelial in origin and their resemblance 

 to ordinary glands is usually only a matter of superficial appearance. 



Fin. 3. A, Diagram of a multi- 

 cellular gland: al., alveolar type; d., 

 duct; ep., general epithelium; t., 

 tubular type. B, The pancreatic duct 

 and associated portion of the duo- 

 denum in the rabbit. 



