i INTERNAL PEOTECTIVE SECEETIONS 5 



which it is possible to obtain in large quantities (e.g. urine, milk, 

 bile), and which have accordingly been the subject of numerous 

 and exhaustive researches. 



The progress of physics again (particularly Dutrochet's theory 

 of the phenomena of diffusion through permeable membranes, the 

 kindred phenomena of imbibition, capillarity, nitration, the 

 modern doctrine of osmosis through semi-permeable membranes, 

 the molecular concentration of solutions, the isotonicity of animal 

 fluids) has stimulated physiologists in the task of reducing the 

 phenomena of secretion as far as possible to common mechanical 

 principles. In this laudable attempt Ludwig is pre-eminent. 

 As we shall see, he founded a mechanical theory of renal 

 secretion that still holds its own in physiological text -books, 

 and accounts for the fundamental phenomena that accompany the 

 formation of urine. Generally speaking, however, it must be 

 admitted that in the actual state of science we are very far from 

 any mechanical concept of the secretory processes taken as a 

 whole. All the forces brought into play with this object are 

 confronted by the enigma of the metabolic activity of the living 

 cell, so that the teaching of Joh. Miiller stands firmer nowadays, 

 after all the vigorous attempts that have been made to overthrow 

 it, than in 1830 when it was first formulated. 



II. The simplest glands, from both the morphological and the 

 physiological point of view, are represented by epithelial tissues, 

 which form alveoli or perfectly closed spaces, i.e. are destitute of 

 excretory ducts by which the secretion is poured out either to 

 the cutaneous surface, or to the inverted mucous surfaces (of the 

 digestive tube, respiratory passages, genito-urinary apparatus). 

 Since all external secretion is excluded by the absence of excretory 

 ducts, it is evident that these closed glands are capable of 

 internal secretion only. Their secretions collect in the glandular 

 spaces, and in proportion as they acquire a certain tension pass 

 through the pores or interepithelial spaces into the periglandular 

 lyrnph spaces, or are directly absorbed by the network of blood 

 capillaries that surrounds the epithelial layer. 



These closed glands have therefore a structure and in all 

 probability a function highly similar to that of the lymphoid 

 tissues and organs discussed in the last chapter. Morphologically, 

 they differ only in having epithelial cells as their essential 

 substrate, while they do not communicate directly, and are not 

 intercalated along the lymph and blood paths, but form quite 

 distinct glandular spaces : physiologically, they differ because ' 

 they do not contribute to the formation of the primary cytological 

 and chemical elements of the blood and lymph, but represent 

 special factors which modify the constitution of these two fluids, so 

 as to adapt them to the normal life of the body as a whole. 



The recognition of the vast importance of the secretory function 



