136 MANUAL OF PHYSIOLOGY. 



the same animal. So far as regards the bloodvessels, then, the 

 chorda is directly opposed to the sympathetic. To explain this 

 antagonism we may either assume the existence of local nerve 

 centres governing the muscular coats of the arterioles, and sup- 

 pose that the sympathetic stimulates and the chorda inhibits the 

 activity of these centres, or, .what seems more simple, in the ab- 

 sence of anatomical evidence that such a centre exists, we may 

 attribute to the arterial muscle cells themselves an automatic 

 tonic power of contraction which can be increased by the sym- 

 pathetic and diminished by the chorda tympani. It is singular 

 that if all the nerves leading to the gland be cut, after some 

 hours a pretty copious watery secretion of saliva occurs and lasts 

 for some weeks, after which the cells undergo atrophic changes, 

 and the gland becomes reduced in size. The explanation of the 

 appearance of this so-called " paralytic saliva " is not clearly 

 made out. Possibly the removal of some trophic nerve-influences 

 induces abnormal nutritive changes which cause stimulation of 

 the cells, and ultimately lead to their degeneration. 



The histological investigation of the elements of these glands 

 in the various stages of secretion throws considerable light on the 

 behavior of the cells during their periods of activity and rest. 



It is now certain that the different stages are accompanied by 

 constant structural changes in the cells, which doubtless are inti- 

 mately connected with secretory activity. During the period of 

 rest, that is, the time when the gland is not discharging its secre- 

 tion, the cells slowly undergo a change in their appearance, which 

 is obvious in proportion to the ease with which the material they 

 secrete is recognized in the protoplasm of a cell. Thus, in mucous 

 glands, or in mucus-yielding salivary glands, the changes are 

 conspicuous ; whilst in those which give a watery sacretion they 

 are less easily seen. . 



As an example we may take a mucous gland, such as the orbital 

 gland of the dog, and follow the changes which occur in its cell 

 from the time it ceases to discharge its function. (Fig. 66.) 



Immediately after the prolonged and active discharge of the 

 secretion of the gland, the cells have all the characters of ordinary 

 protoplasmic units, and the distinction between the polygonal cells 



