The Efferent Nerve Fibres 39 



secretory cells, and may thereby modify the secretion. An acceler- 

 ated flow of saliva may be due to contractions of myoepithelial cells. 

 Vascular changes, caused by impulses in vasomotor nerves, may 

 affect the secretion. Conversely, it is conceivable that secretory 

 activity may change the flow of blood through the gland. The 

 possibility of a complicated interaction between secretory, motor 

 and vasomotor processes has rendered investigations on the differ- 

 ent types of fibres present in the nerves difficult. 



THE EFFERENT NERVE FIBRES OF THE GLANDS 



Ludwig (185 1 ) discovered that saliva flowed from the submaxil- 

 lary gland on stimulation of the chorda-lingual nerve even if the 

 nerve had been cut centrally to the electrode ; and although it had 

 been fully accepted, for more than a century, that salivary secretion 

 is caused by the brain acting in some way on the glands through 

 nerves, this experiment by Ludwig has been regarded as the first 

 definite proof of the existence of special secretory nerve fibres. 

 The discovery of vasodilator nerve fibres was made in the same 

 preparation (Claude Bernard, 1858). The hyperaemia in the gland, 

 caused by stimulation of the chorda-lingual nerve, was, indeed, so 

 impressive that an old hypothesis was revived according to which 

 the flow of saliva is due to increased filtration pressure in the gland. 



It has, on the other hand, been repeatedly suggested that the 

 vasodilation following stimulation of the parasympathetic nerve 

 might result from excitation of secretory, and not vasodilator fibres, 

 the increased blood flow being due to agents released from the 

 activated secretory cells (Barcroft, 1914; Hilton and Lewis, 1955). 



When the sympathetic innervation of the salivary glands is con- 

 sidered, similar lines of thought are encountered. On stimulation 

 of the cervical sympathetic trunk Ludwig (1851) observed a flow 

 of saliva from the submaxillary gland and Claude Bernard (1858) 

 an intense vasoconstriction. Later investigators have noticed periods 

 of vasodilation, alternating with constriction or following it when 

 the stimulation is discontinued. These findings have not, however, 

 been taken as evidence by all investigators that secretory, vaso- 

 constrictor and vasodilator fibres are present in the sympathetic 

 innervation of the gland. Most, but not all, workers seem to agree 

 that the vasodilation is secondary to the secretion and not due to 

 activation of separate vasodilator fibres. The hypothesis has been 

 proposed that there are no secretory fibres in the sympathetic 



