304 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



for extirpation of the ganglion of Meckel is without effect on the parotid 

 secretion. As a consequence, it must be concluded that these fibres 

 pass to the lesser superficial petrosal nerve, which anastomoses with 

 the otic ganglion. For it has been found that extirpation of the otic 

 ganglion, or section of the lesser superficial petrosal nerves, arrests sali- 

 vation. According to Heidenhain, the glosso-pharyngeal nerve also fur- 

 nishes secretory fibres to the parotid, the fibres passing from this nerve 

 to the nerve of Jacobsohn, and thence into the lesser superficial petrosal. 

 Relations between the parotid secretion and the excitation of the cere- 

 bral glandular nerves seem to be about the same as for the submaxillary 

 gland. The proportion of solids and salts augments with the intensity 

 of the stimulation, while the proportion of organic matter increases as 

 long as the glands are fresh, but diminishes if they become exhausted. 



The secretory influence of the sympathetic on the parotid has been 

 the subject of considerable controversy ; the general opinion being that 

 the sympathetic influences the parotid secretion only by diminishing 

 the calibre of the capillaries. Certain authors have, however, held that 

 in certain species excitation of the sympathetic produces a temporary 

 increase in the parotid secretion. According to Eckhard, the parotid 

 of the sheep continues to secrete even after section of all its nerves, 

 being thus analogous to the secretion poured out by the salivary gland 

 after section of the chorda tympani. 



From the above facts it appears that the secretion of saliva is com- 

 posed of two phases, the first, a preparatory stage ; the second, the 

 essential stage. 



The preliminary stage of salivary secretion is that of filtration of 

 serum of the blood into the lymph-spaces around the acini of the salivar}^ 

 gland. This act is entirely under the control of the vascular nerves, 

 which, by changing the calibre of the blood-vessels, and by thus 

 increasing or decreasing the pressure within them, facilitate or hinder 

 the transudation of serum. The influence of the circulation on secretion 

 is, therefore, indirect. When the small arteries of the glands dilate 

 more blood passes through them, a larger amount of nutritive material 

 filters through into the lymph-spaces, and is appropriated by the gland- 

 cells, whose vital processes must be thus quickened. 



The second stage is that of true secretion through the action of the 

 gland-cells, and, as has been already shown, is independent of the 

 circulation and is under the control of the secretory nerves. The nature 

 of these changes occurring in the act of secretion within the gland-cells 

 is to a certain extent rendered explainable from the study of the histo- 

 logical changes which occur within the gland-cells. As has been already 

 stated, the salivary glands ma3 r be divided into two types, the serous 

 and the mucous t} r pes. This distinction, which has only as yet been 



