i8o 



COMPARATIVE ANATOMY 



tiates into a secretory portion and a lining for the duct. The secretory 

 cells become intimately associated with blood vessels and nerves. 



Sweat Glands. In man sweat glands occur in most regions of the 

 body, and are especially abundant on the palms and soles. They are, 

 for the most part, of the simple tubular type, much coiled to increase the 

 secreting area; but those of the axillae are branched and greatly enlarged. 

 They are of the "vitally secretory" type, that is, the cell protoplasm 

 merely produces the secretion, but is not converted into it, and the cell 

 continues ahve indefinitely. The sweat is usually oily, but becomes 

 watery under the influence of the nerves. See Fig. 136. 



Fig. 137. — Scheme of different kinds of nipples. Single line, ordinary integument, 

 double line, that of primary mammary pocket. A, primitive condition, found in 

 Echidna; B, human nipple; D, Didelphys before lactation; C, same at lactation E, 

 embryonic, F adult conditions in cow. B and C are true nipples, F a pseudo-nipple 

 (teat). (Based on figures by Weber from Kingsley.) 



Sebaceous Glands. Sebaceous glands in man also occur on most 

 parts of the body, but are wanting on the palms and soles. Most hairs 

 have sebaceous glands connected with their follicles. 



They are of the acinous type, and necrobiotic, that is, their protoplasm 

 forms the fatty secretion, which the cell extrudes, and then dies. 



Other Glands. Besides the sebaceous and sweat glands, there are 

 other highly speciaHzed cutaneous glands, the lacrimal glands of the eye 

 and the Meibomian of the eyelids, the wax glands of the auditory meatus, 

 besides preputial, vaginal, anal, and mammary glands, which occur in 

 most mammals. 



Of the organs which have evolved from glands, none are more sur- 

 prising than the luminous organs or photophores of deep sea fishes. These 

 are true dark lanterns since they have a condensing lens and a reflecting 



