Sympathetic Innervation 49 



tain fluid; the arguments for a motor innervation have been of the 

 following type : the responses often decrease on repeated stimula- 

 tion of the sympathetic fibres, and can be increased by blowing 

 fluid backwards through the duct towards the gland or by previous 

 stimulation of the parasympathetic fibres. Through the last-men- 

 tioned observation the problem of the motor innervation has been 

 linked with that of "augmented salivary secretion". Experiments 

 by Bradford (1888) and Langley (1889) showed that the response 

 to sympathetic stimulation could be increased by previous excita- 

 tion of the parasympathetic fibres. Langley's explanation was that 

 the first stimulation left the gland cells in a state of heightened 

 excitability, which persisted for a short period. Numerous later 

 investigators have accepted this mechanism as responsible for the 

 "augmented secretion", others have emphasized the importance 

 of a contractile mechanism, activated by the sympathetic fibres and 

 usually presumed to be identical with the myoepithelial "basket 

 cells". Babkin (1950), who with co-workers has contributed much 

 to the solution of the problem of the "augmented secretion", has 

 given an exhaustive review of the literature. From this it is apparent 

 that both mechanisms may be at work. The sympathetic nerves of 

 the salivary glands obviously contain motor fibres acting on con- 

 tractile elements. These may also be activated by injected adren- 

 aline or histamine. Most investigators, collecting saliva from a 

 cannula in a salivary duct, have probably noticed that if some 

 saliva, secreted after sympathetic stimulation or injection of adren- 

 aline, is left on the tip of the cannula, it will slowly disappear, 

 sucked back into the gland. A reasonable explanation seems to be 

 that this occurs because the contracted elements slowly relax, 

 although it could be due to reabsorption in the ducts. 



The effect of the sympathetic fibres on the contractile elements 

 has been well illustrated in experiments on the sheep's parotid 

 gland. Histological investigations have shown this gland to contain 

 myoepithelial cells (Silver, 1954), and stimulation experiments 

 have made it likely that these can be excited by sympathetic stimu- 

 lation or injection of oxytocin (Kay, 1954). In the experiment of 

 Fig- 3-3 (Coats, Denton, Goding and Wright, 1956) the effect of 

 sympathetic stimulation is demonstrated. The parotid gland of the 

 sheep is in permanent activity even if denervated, presenting an 

 example of spontaneous secretion. This process continuously fills 

 the ducts of the gland, and saliva is discharged at a constant rate, 

 p.s.g. — E 



