Parasympathetic Secretory Innervation 47 



As to the quantitative aspect, the following may be pointed out. 

 In the perfusion experiments, quoted above, about 0-2 //g of acetyl- 

 choline was obtained from the cat's submaxillary gland during a 

 stimulation period of two minutes, of which two-thirds should 

 originate from the postganglionic endings in contact with the 

 secretory cells. The secretory threshold to acetylcholine injected 

 through the lingual artery in retrograde direction, all arterial 

 branches except that of the submaxillary gland having been tied, 

 was found to be o-i fig (Graham and Stavraky, 1953); under 

 similar conditions Gibbs (1935) saw an effect with as little as 

 o-oi fig of acetylcholine, and Beznak and Farkas (1936-37) with 

 o-oo 1 -o- 1 //g. When acetylcholine was injected through the sub- 

 maxillary duct towards the gland the threshold dose was 0-5-1 fig, 

 or 0-1-0-5 fig after previous section of the chorda (Emmelin, Muren 

 and Stromblad, 1954). Considering the unfavourable conditions of 

 these various experiments, which probably allowed only a small 

 fraction of the administered drug to reach the gland cells (for 

 instance, no anticholinesterase was given), it seems safe to conclude 

 that the amounts of the drug released on chorda stimulation are big 

 enough to excite the gland cells. 



It is, in fact, probable that acetylcholine is liberated with a big 

 margin of safety, in the synapse, at the postganglionic endings, or 

 at both sites. This is suggested by the following observation (Em- 

 melin, Muren and Stromblad, 1957a). After section of the chorda, 

 the cut nerve fibres were allowed to regenerate, or hypoglossal 

 fibres were directed towards the synapses. In some cases seemingly 

 fairly normal conditions were restored ; stimulation of the regener- 

 ated fibres caused a lively secretion of saliva, the gland was not as 

 atrophied as a denervated gland and the supersensitivity to chemi- 

 cal agents, characteristic of denervation, had decreased. However, 

 on continued stimulation the rate of secretion tended to decline, 

 and hexamethonium had an abnormally high blocking effect. 



The safety factor is apparently high in another step of the trans- 

 mission mechanism as well ; cholinesterase activity can be greatly 

 inhibited without seriously affecting transmission (Riker and 

 Wescoe, 1949; Dirnhuber and Evans, 1954; Stromblad, 1957a). 



The supposition that acetylcholine normally appears in saliva 

 (Seeker, 1934) is apparently erroneous (Feldberg and Guimarais, 

 1935; Gibbs, 1935; Larson, 1935; Winterstein and Ozer, 1948). 

 Acetylcholine is obviously very effectively dealt with by the esterase 



