754 SPECIAL PHYSIOLOGY. 



ences between the several secretions, cannot depend on the character 

 of the blood distributed to the respective glands, which is uniformly 

 pure arterial blood. Neither can it depend upon the number or ar- 

 rangement of the capillary vessels, for these peculiarities can only 

 determine the quantity, not the quality, of a given secretion. Nor is 

 there any evidence to show that the walls of the capillaries differ in 

 different glands, nor even the basement or limiting membrane, which 

 always presents a glass-like, structureless, appearance. Again, the 

 relative simplicity, or complexity, of a gland cannot be supposed, in 

 any way, to determine the character of its secretion ; for, though dif- 

 ferently formed glands, supplied by the same blood, often yield differ- 

 ent secretions, yet there are cases, in which very similarly formed 

 glands produce different secretions, as, for example, the salivary, pan- 

 creatic, and mammary glands. Moreover, on regarding the Animal 

 Kingdom generally, it is found that similar secretions, as, e. g., the 

 bile, the gastric secretion, and indeed nearly all the secretions, are 

 formed, in different cases, by glands of variable structure, sometimes 

 complex, sometimes simple, according to the position of the animal in 

 the scale of organization. There is one component, however, of all 

 secreting and excreting organs, whether membranous or glandular, 

 viz., the epithelial layer, which appears to be essential to specific 

 secretion, and to be the seat of the selective assimilative power of the 

 true secreting glands, and of the selective eliminative power in the 

 excretory glands. The epithelial cells of different membranes and 

 glands, most frequently present differences of structure and arrange- 

 ment, suggestive of the possession of different properties. The peptic 

 and hepatic cells, the columnar cells of the intestinal tubuli, and the 

 cells of the sebaceous cutaneous glands, are totally different from 

 each other. Even in the simplest glands in animals, as in the so- 

 called hepatic tubuli, special epithelial cells are discernible. Epi- 

 thelial cells are components of the solid texture of the body, subject 

 to the ordinary processes of development, growth, and nutrition ; but 

 they are distinguished by a peculiar destiny or purpose in the 

 economy, and to them we must refer that special form of nutrition, 

 which, instead of resulting in the development or maintenance of a 

 tissue, destined for certain mechanical or vital purposes in the animal 

 framework, is employed for the formation, or separation, of more or 

 less liquid products, intended for digestive or other uses, or destined 

 to be eliminated, and entirely discharged, from the system. 



Most frequently, the gland-cells effect changes in the materials 

 which are presented to them by the blood ; but, at other times, they 

 attract from that fluid compounds which pre-exist in it. In either 

 case, it is these cells which attract or separate the products or educts 

 from the circulating fluid. 



The general conditions which influence the functions of secretion 

 and excretion, are the quantity of blood supplied to the respective 

 glands, the quality of that blood, the presence of external stimuli, act- 

 ing directly or indirectly on the nerves, and, perhaps, some governing 

 influence of the nervous system itself. 



As a rule, an increased quantity of blood supplied to a gland, de- 



