HISTOLOGY 139 



The simple external epithelium of the earthworm and the stratified 

 epidermis of a fish combine cuticular, glandular and sensory speciahzations. 

 CiHa and mucous glands commonly occur together in the same epithehum. 

 An epithelium, either simple or stratified, may be rendered sensory by the 

 presence of free nerve terminations, that is, the terminal twigs of a nerve 

 fiber ramifying amongst the epithelial cells (Fig. loi). These nervous 

 structures, however, are not produced by the epithehum itself but invade it 

 from adjacent tissue. The term "sensory epithehum " should be restricted 

 to those whose speciahzed sensory structures 

 are certain of the cells of the epithehum. ''^^^^'^ — ^^^^^^^^?^^^^^^^^~"^= ^__^ 



The name "gland" is apphed to various \ }r\ y i r^h{f'''f\)jj^ 



organs which have Httle in common beyond rS^T^f^^^ 



a superficial similarity which may cause a S^-I^.y'^ ' ' ' ; .- ] \ \\ 



lymph gland to be mistaken for a salivary ( JH^ ' \ i 



gland. In general, glands produce or dis- \ ' 

 charge something. But organs whose prod- Fig. ioi.— Free nerve ter- 



,... . J • •£: mination in the epidermis of 



ucts are as different m nature and signifi- saiamandra. (From Kingsiey, 

 cance as are sweat, eggs and blood cells after Retzius.) 

 hardly merit the same name. Accepting 



the name, it is at once necessary to distinguish the following funda- 

 mentally different types of glands: (i) secretory glands whose prod- 

 ucts are retained, at least temporarily, and serve the animal in some 

 useful way — e.g., mucous, sahvary and thyroid glands; (2) excretory 

 glands which ehminate waste from the body — the kidneys; (3) cjrtogenic 

 glands whose products are Hving cells — the reproductive glands which 

 produce sperm cells and egg cells, and the various lymph and hemal 

 glands, including the spleen, which produce white blood cells. The prod- 

 ucts of these cell-producing glands may either be retained to serve useful 

 purposes as are secretions, or discharged as are excretions. Blood cells 

 are retained, but reproductive cells are discharged by means of ducts which 

 are identical with or closely related to those which discharge waste from 

 the kidneys. 



Secretory glands are of most diverse sorts. The simplest and presum- 

 ably primitive type of gland is the unicellular gland of a glandular 

 epithelium. The association of numerous secreting cells together to 

 form a glandular organ provides for a more efificient carrying on of 

 the secreting process. Such an organ is called a multicellular gland 

 (Fig. 94, g). 



Nearly all multiceUular secretory glands arise directly from epitheha 

 and retain their epithelial character. Certain of the endocrine glands, 

 whether arising directly from epithelium or not, are epithelioid. 



