758 SPECIAL PHYSIOLOGY. 



the air, but also in regard to some of the products of oxidation of the 

 albuminoid tissues, viz., urea, ammonia, and carbonic acid. 



Different secretions and excretions differ as to the time at which 

 they are prepared. Some are constantly or continuously formed, 

 whilst others are secreted, or excreted, intermittently, or'remittently. 

 Secretions are more commonly intermittent or remittent, serving occa- 

 sional purposes in the economy, as, e. g., the gastric juice, the secre- 

 tion of which is probably limited to the period of digestion, and the 

 lachrymal, salivary, hepatic, pancreatic, and mammary secretions, 

 which are always being secreted in small quantities, but are, from 

 time to time, as required, produced in much larger amounts. On the 

 other hand, the excretions being injurious, and requiring to be elimi- 

 nated as rapidly as possible from the blood, are characterized by 

 their constant separation, both day and night, the process varying in 

 activity, however, according to circumstances. 



The force by which the secretions are urged along the ducts, is 

 probably the vis a, tergo, dependent upon the pressure of continuously 

 fresh-formed portions of fluid secreted in the commencing ducts. The 

 movement in the larger ducts is aided by the slow contraction of the 

 organic muscular fibres in the walls of the ducts. In some cases, as 

 in the bile and pancreatic ducts, and the ureters, rhythmic movements 

 have been seen, and in others, peristaltic movements. The pressure 

 on the fluid in the ducts is sometimes considerable, as is seen by the 

 occasional ejaculation of the saliva, and the expulsion of the milk. 

 The action of the surrounding muscles must, here and elsewhere, also 

 be taken into account. In certain cases, the larger ducts are di- 

 lated, near their mouths, into temporary receptacles for the secretion, 

 as is seen in the parotid and lactiferous ducts. Still more special de- 

 velopments of the excretory apparatus are met with in the shape of 

 reservoirs or bladders, with contractile walls, when as in the case of the 

 bile, the secretion is abundant and used intermittently, or when, for 

 other reasons, an excretion requires to be retained, and only occa- 

 sionally expelled. 



The daily quantities of the various secretions and excretions, as 

 stated elsewhere in the account of each, differ remarkably in different 

 individuals, and in the same individual under different circumstances. 

 The quantities of the excretions, in health, conform to the quantity of 

 water taken in the solid and fluid food, one of the objects of this elimi- 

 nation of water, being to maintain the due characters of the blood. 

 In the formation of the extraordinary quantities of the secretions em- 

 ployed in the digestive process, the water concerned is, as already 

 mentioned, separated from the blood, used in dissolving the food, re- 

 absorbed, and re-secreted many times over. 



The preceding facts and considerations illustrate the general re- 

 semblances and differences between nutrition and secretion. In both 

 processes, the blood yields a common plasma to certain organs ; from 

 this plasma, in both, materials are attracted by a selective property 

 possessed by pre-existing tissue elements ; and, in both, the residual 

 and altered plasma re-enters the blood. But this difference arises: in 

 the one, the separated materials form an intrinsic 'part of a solid and 



