250 STUDIES IN ADVANCED PHYSIOLOGY. 



the physical process of osmosis. The influence of the 

 nerves on secretion was then demonstrated in vivisections 

 made by Ludwig in 1851, who by electrically stimulating 

 the cerebral nerve going to the submaxillary of the dog 

 caused a secretion of saliva, which rose to the height in the 

 tube of 200 millimeters of mercury, while the height of the 

 blood in the carotid artery rose to 112 millimeters of mer- 

 cury only. The action of the sympathetic nerve on secretion 

 and the changes of the vascular supply of glands were in 

 1858 discovered by Claude Bernard. Finally the micro- 

 scopist, Heidenhain, made a series of researches on the 

 histological changes in secreting cells, which have demon- 

 strated that the secreting cells themselves are the seat of 

 active chemical changes which form the secretions. 



SECEETION. 



The term "secretion" is frequently made to apply to a 

 varying number of things. Sometimes all the substances 

 which are given off by the blood are called secretions. This 

 would make the lymph which oozes out of blood capillaries 

 a secretion, and would also make the gases which figure in 

 respiration secretory products. This use of the term secre- 

 tion is, however, much too wide, for physiologically speak- 

 ing, lymph and the blood gases do not figure in any way as 

 the product of glands proper. In the second place, the 

 term "secretion" is made to apply to all the discharges of 

 all the various glands. There is no special objection to this 

 application of the term. Other physiologists, however, speak 

 of secretions and excretions, calling "secretions" those 

 products of the glands which are intended for a further use 

 in the body, and apply the term "excretions" to. those 

 glandular discharges which are intended for no further use, 

 but are simply to be thrown off. But even these uses of 

 the term are not satisfactory, because it is not always easy 

 to tell whether a certain secretion is directly intended for 

 further use, or is a mere waste product to be eliminated. 

 To do away with such an ambiguity, therefore, the term 



