1912.] Relation between Secretory and Capillary Pressure. 315 



cells are contractile. The membrana propria clearly acts to the secreting 

 cells as does the sarcolemma to the muscle fibre. The cells stimulated to 

 secretory activity imbibe water from the capillaries and extrude the 

 salivary secretion into the ductules at a pressure which may double that 

 of the arterial pressure. Meanwhile the capillaries are protected from 

 occlusion by the membranse propriae of the alveoli, which, acting like the 

 leather case of a football or the pericardium of the heart, limit the expansion 

 of the alveolar cells. When the secretion is obstructed, the whole gland 

 becomes tense, the veins are diminished in volume, the circulatory pressure 

 in them is raised until the vessels, arteries, capillaries, and veins approxi- 

 mate to a rigid system at arterial pressure, with a fast rate of flow. As the 

 alveoli swell, this favourable condition of the circulation is at first estab- 

 lished ; finally, however, the vessels may be so far narrowed that the flow 

 becomes lessened, but not stopped, owing to the restraining action of the 

 inembranre propria?. It seems to us probable that the slackening of the 

 circulation may be due to the leakage of saliva into the intra-alveolar 

 connective tissue. If we cease to excite the chorda, the secretory pressure 

 drops slowly, owing to such leakage. The basket cells may possibly, by 

 their contractile power, help to squeeze the saliva into the ducts. "We 

 have no evidence to offer as to such a function. No doubt the membranee 

 proprise may be supported by the strands of connective tissue which 

 surround and knit together the alveoli. 



We believe that we are here dealing with a fundamental principle in 

 the construction of many parts of the body. The cells in the secreting 

 glands are enclosed on one side by a cuticular membrane which permits their 

 imbibition of fluid but checks their power to swell, thus enabling them to do 

 work and at the same time receive an ample supply of blood. The 

 mechanism reminds us of Pfeiffer's semi-permeable membrane, with this 

 difference, that the living membrane can vary its permeability. The muscle 

 fibres are enclosed by a sarcolemma, the nerve-fibres by a neurilemma, the 

 secreting alveoli by a membrana propria. In each case the membrane may 

 limit the swelling of the protoplasmic content, and so permit a high 

 imbibition pressure within. Turning to the kidney, we find a similar 

 membrana propria enclosing the secreting cells, both of the capsules and of 

 the tubules. The secretory pressure in the obstructed ureter, however, does 

 not rise so high as the arterial pressure. Starling gives the following 

 readings :— * 



* Starling, ' The Fluids of the Body,' 1909, p. 110. Constable, London. 



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