314 



Messrs. L. Hill and M. Flack. 



[Apr. 23, 



feels tense to the touch. By squeezing the gland we find that we can further 

 raise the pressure of the secretion, while at the same time we impede the 

 outflow of blood. On allowing the secretory pressure to fall to atmospheric 

 pressure we find that the outflow from the vein becomes much ampler; in one 

 experiment it was approximately doubled, e.g. 40 drops in 15 seconds as 

 compared with 27 drops in 20 seconds at the highest secretory pressure 

 (240 mm. Hg.). In some experiments we observed the venous outflow 

 from the time we began to excite the chorda tympani. Under these 

 circumstances we find that the outflow from the gland at rest is very slow and 

 the blood venous in colour. On stimulating the chorda, the outflow becomes 

 very ample, the blood arterial in colour, and remains so during the rise of 

 secretory pressure until this pressure rises higher than the arterial, when the 

 outflow becomes lessened and the blood less arterial in colour. 



Our results show clearly that there exists in the salivary gland a structural 

 mechanism which protects the capillaries from the maximum secretory 

 pressure which may exist within the alveoli and ducts of the gland. The 

 gland is of the compound racemose type, and composed of alveoli around 

 which a dense network of capillaries is spun. The alveolar cells, 2-3 in 

 diameter, are formed of bioplasm, which probably contains some 80 per cent, 

 of water. The wall of the capillaries is formed of a single layer of exceed- 

 ingly thin cells, less than 1 ^ in thickness. It does not seem possible to us 

 that either the alveolar or the capillary cells can support any difference of 

 fluid pressure. The blood, tissue, lymph and cell protoplasm must all be at 

 one and the same fluid pressure, unless there exists some rigid supporting 

 structure, which surrounds the alveoli and limits their expansion. On 

 seeking for histological evidence, we find that Kolliker* and other 

 histologists describe the alveoli as surrounded by a thin structureless 

 membrane, the membrana propria. This can be easily demonstrated in 

 glands which have been treated with strong potash. The membrana propria 

 is thus shown to be a structureless membrane less than 1 fi thick, and 

 wonderfully strong. Kolliker states that, upon injecting the ducts of the 

 glands under high pressures, the secretory cells may be destroyed or 

 separated from the membrana propria, but that this membrane seldom 

 becomes torn. Before this happens, extravasations will take place through 

 the walls of the ductules. We ourselves have seen such extravasations in 

 glands which have been kept secreting for a considerable time at a high 

 pressure. The so-called basket cells also surround the gland cells, there 

 anastomosing by their branches to form a kind of protoplasmic framework, 

 which contains the secreting cells. It has been suggested that the basket 

 * Kolliker, ' Handb. der Gewebelehre des Menschen,' Leipzig, 1899, vol. 3, p. 46. 



