14 PHENOMENA OF SALIVARY SECRETION. [BOOK II. 



centres exert on the nerve fibres connected with them and on the tissues 

 and organs which are innervated by them. Thus the Gasserian ganglion 

 is said to efcert a trophic influence on the eye, as its complete division 

 leads in general to destructive changes in that organ; or to take a case less 

 open to controversy, the ganglion on the posterior root of a spinal nerve 

 is said to exert a trophic influence on the afferent fibres connected with 

 it, because when these are separated from it, and so removed from its 

 influence, they undergo a process of degeneration. 



Vascular Changes which accompany Secretion. 



Ordinarily when a salivary gland passes from the state of rest into 

 that of activity it is at once the seat of an increased blood-flow, which 

 is associated with the dilatation of the blood-vessels of the organ. 

 Under these circumstances the blood leaving the gland presents a 

 florid arterial, instead of a venous colour, which characterises that of 

 the organ when at rest. This vascular dilatation is explained by the 

 coming into action of the before-mentioned vaso-dilator fibres ; it is 

 not necessarily dependent on the act of secretion, as it may occur after 

 the secretory fibres have been paralysed. 



Heat evolved during Secretion. 



As was shewn in a now classical investigation of Ludwig 1 , when 

 the salivary glands are thrown into activity there is a rise in tem- 

 perature, so that the temperature of the saliva leaving the sub- 

 maxillary gland may exceed by 1 C. that of the blood flowing to 

 the gland. This rise in temperature cannot be explained by a 

 study of the chemical characters of the salivary secretion, but is 

 doubtless the result of the increased metabolic changes which neces- 

 sarily accompany the act of secretion in the gland cells, and which 

 chiefly affect their protoplasm. 



The Secretion of Saliva not an act of filtration. 



That the secretion of saliva (and indeed that secretion in general) 

 is not a mere act of filtration, was proved by Carl Ludwig when 

 he shewed that saliva can be secreted by a gland though the pres- 

 sure exerted by the secreted fluid in the ducts within it is considerably 

 higher than that of the blood circulating through the arteries which 

 supply it. On many grounds it may be positively asserted that the 

 secreting cells are the primary agents in tbe withdrawal from the 

 blood of tbe water necessary for the secretion, though the exact 

 nature of the process is yet unknown 2 ; similarly on the grounds 

 stated below we know that within the protoplasm of the gland cells 

 the characteristic soluble constituents of the secretion are formed. 



1 Ludwig u. A. Spiess, ' Vergleichung der Warme der Unterkiefer-Driisenspeichel 

 und des gleichzeitigen Carotidenblutes.' Zeitschr.f. rat. Med., 1858, p. 361. 



2 Consult Heidenhain, Hermann's Handbuch, Vol. v. p. 72. 



