INTRODUCTORY 3 



substances are often included in the same category ; such are 

 the calcareous shells of the Foraminifera, the chitinous cases 

 of insects, cell membranes, etc. Many authors would include 

 also certain intercellular substances, such as are found in the 

 fibrillar connective tissue, cartilage, and bone. Where, how- 

 ever, definite morphological structures are formed, it will be 

 well not to employ the term " secretion." Thus, the products 

 of the reproductive organs the ova and spermatozoa are 

 not to be included in this category. 1 



The term " secretion " in higher animals is ordinarily meant 

 to apply to the liquid or semiliquid products formed by the 

 glandular organs, or to the process of manufacture and setting 

 free of such products. The idea of secretion has, from the 

 earliest period of physiology, been associated with what are 

 called " glandular " organs. The term " gland " was applied 

 in the early days of anatomy to a very varied group of struc- 

 tures which resembled each other only in certain general 

 external characters. 2 Thus, in the same general category 

 were included not only such typical glands as the pancreas, 

 submaxillary glands, and kidneys, but also the liver, spleen, 

 lymph nodules, reproductive organs, and, as they were dis- 

 covered, the thyroid body, the suprarenal capsule, the thymus, 

 and so on. 



There is no need to describe a " gland " in any detail. We 

 may define a gland as a structure made up of one or more 

 cells of a special epithelial character which forms a product, 

 the secretion, which is discharged upon an epithelial surface, 

 such as the skin or a mucous membrane. This is the definition 

 of an ordinary or externally secreting gland. 



The simplest type of a gland consists merely of a layer of 

 epithelial cells placed upon a basement membrane, while 



1 The seminal fluid as a whole is, of course, to be looked upon as a secretion 

 coming partly from the testes, but chiefly from the accessory sexual glands. 

 The secretion itself is to be regarded as a vehicle for the transportation of 

 the spermatozoa, and possibly also for their nutrition. But whether or no 

 we view the testis as a gland having an external secretion, we shall see reasons 

 later for considering it to be a gland with an internal secretion. 



2 " Die Classe der Driisen ist eine derjenigen welche eine Wissenschaft in 

 ihrer ersten Jugend leichtsinnig schaff t, und welche zu begrenzen und recht- 

 fertigen ihr in Zeiten der Reife grosse Sorgen und Miihe kostet. Mannhatte 

 anfangs nur die aiissere Form in Augeund nannte jedes weiche, rundliche, 

 gefassreiche und daher rothliche oder rothe Organ eine Druse, und das Gewebe 

 solcher Organe driisig (Henle). 



