756 SPECIAL PHYSIOLOGY. 



all, possess others derived from the cerebro-spinal nervous system. 

 The experiments just referred to, show that the quantity of a secretion 

 is differently affected by the section, or irritation of these two sets of 

 nerves. Thus, irritation of the pneumogastric nerves, increases the 

 quantity of the gastric juice, whilst irritation of the sympathetic 

 nerves, diminishes or arrests it. Again, division of the sympathetic 

 nerves of the submaxillary gland, increases the flow of saliva, but ir- 

 ritation of the distal cut portion of the nerve, diminishes it ; on the 

 other hand, section of the cerebral nerve diminishes, whilst irritation 

 of the distal cut end, augments it. Even simple irritation of the un- 

 divided sympathetic nerves causes diminution, whilst a similar irrita- 

 tion of the undivided cerebral nerve, causes an increase of the secre- 

 tion. Since, in the former case, the small arteries of the gland 

 contract, and the supply of blood is diminished, whilst in the latter, 

 those vessels dilate, and more blood is distributed to the gland, the 

 diminution or augmentation of the secretion accords, in either case, 

 with differences in the quantity of blood conveyed to the gland, and 

 the influence of the nervous system in regulating the quantity of the 

 secretion, is indirectly manifested by the dilatation or contraction of 

 the coats of the small arteries. 



With regard to the influence of the nerves on the quality of a secre- 

 tion, it is found that when the a'rteries are contracted, and the supply 

 of arterial blood lessened, not only is the quantity "of saliva dimin- 

 ished, but the color of the venous blood returning from the gland is, 

 as usual, dark ; whereas, when the arteries are dilated, the supply of 

 blood is increased, and the amount of secretion augmented, then the 

 color of the returning venous blood is bright. In the former case, the 

 passage of the blood through the gland seems to be sufficiently delib- 

 erate to permit of the proper nutritive or secretive interchanges be- 

 tween it and the epithelial cells ; whilst in the latter case, the blood 

 flows so rapidly through the glands as not to undergo these changes. 

 It is not proved that the sympathetic nerve determines, or even in- 

 creases the secreting power of the gland. 



By controlling the quantity and velocity of blood passing through 

 a gland, the sympathetic nerves may, therefore, not directly, but in- 

 directly, by permitting the characteristic function of the gland, pre- 

 serve the essential qualities of its secretion ; whilst, on the other 

 hand, the cerebro-spinal nerves, by determining an increased supply 

 and quicker motion of the blood, must, by partially interfering with, 

 or overwhelming the special actions of the secreting cells, increase the 

 fluid in the secretion, but so dilute and lower its qualities. 



Whilst it is not yet proved that either the sympathetic or the cere- 

 bro-spinal, nervous system has any power over the chemical acts of 

 secretion, independently of their governing influence over the blood- 

 vessels, it must be added that this is a point on which opinions are at 

 variance. As already remarked (p. 705), if, as it seems scarcely pos- 

 sible to doubt, the nervous system influences the secreting processes in 

 such of the lower animals as are unprovided with bloodvessels, and 

 yet possess nerves, it is difficult to deny the existence of some such 

 direct influence in the higher vascular animals arid in Man, however 

 unintelligible the nature of such a controlling process may be. 



