OF SECRETION. 189 



trolling power ; that by keeping parts in states answering to the passive 

 and active conditions of inorganic chemistry, it can suspend the action of 

 the respired oxygen or permit it to take effect. This controlling power 

 is, however, altogether distinct from a generative one, and all the heat dis- 

 engaged is due to oxidation. It is also possible that not only are these 

 states of activity or passivity impressed on the tissues by the agency of 

 the nerves, but also upon the respired oxygen itself, since that gas is no 

 exception to the rule ; it also exhibits allotropism. Its passive state is 

 Priestley's oxygen, its active is Ozone. In its transit from the air-cells 

 into the blood it may experience such a change, and have at once com- 

 municated to it a high degree of activity. 



- 

 CHAPTER XL ; 



OF SECRETION. 



SEROUS, MUCOUS, AND HEPATIC SECRETIONS. 



of Secretion. Type of secreting Mechanism. Filtration and Cell Action. Of Serous 

 Membranes and their Secretions. Of Mucous Membranes and their Secretions. Of Hepatic Se- 

 cretions. The Liver: its Development and Structure. Source, Quantity, Composition, Uses, 

 and Flow of the Bile. Existence of biliary Ingredients in the Blood. Production of Sugar and 

 Fat in the Liver. Changes of the Blood-cells in it. General Summary of the four-fold Action 

 of the Liver: it produces Sugar and Fat, eliminates Bile, is the Seat of the final Destruction 

 of old Blood-cells, and of the Completion of new Ones. Of the ductless Glands. The Spleen: 

 its Functions. 



Two classes of substances occur in the blood the products of decay 

 and the elements of nutrition. The equilibrium of the system requires 

 that the former should be removed and the latter appropriated. 



The primary object of the function of secretion is this dismissal and 

 appropriation, and therefore, through the latter duty, secre- object of secre- 

 tion becomes connected with nutrition. tion - 



The elementary type of a gland or organ of secretion consists ot a sac, 

 on the interior of the wall of which a network of arterial ramifi- Type of a 

 cations is spread ; this delivers its blood into a similar network e land - 

 of veins. The matter which the gland is destined to separate oozes 

 from the arterial capillaries into the interior of the sac, and is delivered 

 through the neck or mouth thereof, which may be spoken of as the duct. 

 It will be presently shown that the material which thus finds its way 

 into the interior of the sac is not fabricated by that organism, but is 

 brought to it pre-existing in the affluent current of arterial blood. As 

 our knowledge of the functions of glandular structures becomes more 



