GLANDS 261 



secreting epithelium and which present a more or less distinct tubular 

 arrangement. These bodies are the adrenals, thyroids, parathyroids, 

 carotid glands, coccygeal gland, paraganglia, hypophysis cerebTi, ~and 

 epiphysis cerebri (pineal gland). These are also known as organs of 

 internal secretion. 



While these glands do not possess an excretory duct, nevertheless 

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

 which find their way into the blood or lymph as so-called 'internal se- 

 cretions? 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 epithelial parenchyma. In some instances lymphatics 

 are distributed in a similar manner within the gland. 



The property of internal secretion is not peculiar to the ductless 

 glands. It has long been ascribed to the liver cells in connection 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 economy in certain ways which can 

 not be accounted for by the properties of their external secretions. 



Finally, it must be emphatically stated that the types of secreting 

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

 many forms will be found which might well be placed under either 

 of two or more types. Hence any classification of secreting glands be- 

 comes more or less arbitrary; nevertheless such a classification is of 

 extreme importance as serving to establish in the mind of the student 

 certain typical pictures with which individual glands may be compared, 

 and important structural details will thus be noticed which might other- 

 wise escape observation. 



