SECRETING GLANDS 195 



During the period of their inactivity the lining epithelial cells are 

 much flattened and the acini appear shrunken. The epithelium 

 of the lactating gland, on the other hand, is cuboidal or columnar, 

 the height being more or less dependent upon the accumulation 

 of secretion within the cell. 



The secretion is formed in the same manner as in the tubular 

 glands with an additional process of fatty infiltration by which fat 

 droplets are formed within the cytoplasm. These droplets collect 

 in the central portion of the cell and are finally discharged into 

 the lumen of the acinus with apparent rupture of the cell mem- 

 brane and the escape of a portion of its superficial cytoplasm. 

 The epithelium is thus capable of repeated secretion. 



The mammary glands may be considered as offering an inter- 

 mediate type between the branched saccular and the tubulo-acinar 

 types. - 



Ductless Glands. Under the head of secreting glands it is nec- 

 essary to consider certain structures which apparently contain 

 secreting epithelium and which present a more or less distinct 

 tubular arrangement. These bodies are the adrenals, thyroids, 

 parathyroids, carotid glands, coccygeal gland, and hypophysis 

 cerebri. 



While these bodies do not possess an excretory duct, neverthe- 

 less some of them certainly, and the others probably, form certain 

 products which find their way into the blood or lymph as so-called 

 "internal secretions.''' The epithelium of the glands may form 

 either alveoli, tubules, or solid cell columns, which are supported 

 by very delicate connective tissue tunics. Many blood vessels, 

 often of the thin walled sinusoidal type, are found within these 

 tunics and are thus brought into intimate relation with the epi- 

 thelial parenchyma. In some instances lymphatics are distributed 

 in a similar manner within the gland. 



The property of internal secretion is not peculiar to the duct- 

 less glands. It has long been ascribed to the liver cells in con- 

 nection with their influence upon nitrogenous and carbohydrate 

 metabolism, and, in fact, many secreting glands, even though not 

 of vital importance, are nevertheless found to influence the econo- 

 my in certain ways which can not be accounted for by the proper- 

 ties of their external secretions. 



Finally, it must be emphatically stated that the types of secret- 

 ing glands, as above described, are not bound by hard and fast 



