



SECRETION. 171 



tain time, one to three days, begins to secrete slowly and the secretion contin- 

 ues uninterruptedly for a long period-ras long, perhaps, as several weeks and 

 eventually the gland itself undergoes atrophy. Langley l states that section of 

 the chorda on one side is followed by a continuous secretion from the glands 

 on both sides ; the secretion from the gland of the opposite side he designates 

 as the antiparalytic or antilytic secretion. He believes that this continuous 

 secretion is due to the fact that the irritability of the nerve-cells in the secretion 

 centre (see below) in the medulla, as well as of the nerve-cells in the gland 

 itself, is so much increased that the venosity of the blood itself is sufficient to 

 throw them into continuous activity. It is difficult, however, to understand 

 why section of the chorda should have any such effect as this upon the medul- 

 lary centre, especially as it is known that section of the secretory fibres in the 

 sympathetic does not give a similar result. A more plausible explanation is 

 the one suggested by Bradford, 2 namely, that the salivary glands receive through 

 their cerebral nerves certain fibres which may be called anabolic, whose action 

 is to cause suspension or inhibition of the katabolic changes in the gland-cells 

 probably, according to Bradford, by acting on the local nerve-ganglion cells in 

 the gland. When these fibres are removed by section there is nothing to 

 hold the katabolic processes in the gland in check, and as a result we get a 

 continuous secretion and a wasting of the gland. 



Normal Mechanism of Salivary Secretion. Under normal conditions 

 the flow of saliva from the salivary glands is the result of a reflex stimulation of 

 the secretory nerves. The sensory fibres concerned in this reflex must be 

 chiefly fibres of the glosso-pharyngeal and lingual nerves supplying the mouth 

 and tongue. Sapid bodies and various other chemical or mechanical stimuli 

 applied to the tongue or mucous membrane of the mouth will produce a flow 

 of saliva. The normal flow during mastication must be effected by a reflex of 

 this kind, the sensory impulse being carried to a centre and thence transmitted 

 through the efferent nerves to the glands. It is found that section of the 

 chorda prevents the reflex, in spite of the fact that the sympathetic fibres are 

 still intact. No satisfactory explanation of the normal functions of the secre- 

 tory fibres in the sympathetic has yet been given. Since the flow of saliva is 

 normally a definite reflex, we should expect a distinct salivary secretion centre. 

 This centre has been located by physiological means in the medulla oblon- 

 gata ; its exact position is not clearly defined, but possibly it is represented by 

 the nuclei of origin of the secretory fibres which leave the medulla by way of 

 the facial and glosso-pharyngeal nerves. Owing to the wide connections 

 of nerve-cells in the central nervous system we should expect this centre to be 

 affected by stimuli from various sources. As a matter of fact it is known that 

 the centre and through it the glands may be called into activity by stimula- 

 tion of the sensory fibres of the sciatic, splanchnic, and particularly the vagus 

 nerves. So, too, various psychical acts, such as the thought of savory food and 

 the feeling of nausea preceding vomiting, may be accompanied by a flow of saliva, 



1 Proceedings of the Royal Society, London, 1885, No. 236. 



2 Journal of Physiology, 1888, vol. ix. p. 2S7. 



