230 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ues uninterruptedly for a long period — as 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 botli sides ; the secretion from the gland of the opposite side he designates 

 :i- tin' antiparalytic or antilytic secretion. After section of the chorda the 

 iurve-fibres peripheral to the section degenerate, the process being com- 

 pleted within a few days. These fibres, however, do not run directly to the 

 gland-cell ; they terminate in end-arborizations round sympathetic nerve-cells 

 placed somewhere along their course, in the sub-lingual ganglion, for instance, 

 or within the gland substance itself. It is the axons from these second nerve 

 units that end round the secreting cells. Langley 2 has accumulated some 

 facts to show that within the period of continuance of the paralytic secretion 

 (5 to 6 weeks) the fibres of the sympathetic cells are still irritable to stimula- 

 tion. He is inclined to believe therefore that the continuous secretion is due 

 to a continuous excitation, from some cause, of the local nervous mechanism 

 in the gland. On the other hand, it is possible that the mere cessation of the 

 normal action of the chorda fibres is followed by an altered metabolism in the 

 gland cells of such a nature as to cause a continuous feeble secretion. 



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 secretorv 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 

 mi' 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 trans- 

 mitted 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 

 secretorv fibres in the sympathetic has yet been given. Various authors have 

 suggested that possibly the three large salivary glands respond normally to 

 different stimuli. This view has lately been supported by Pawlow, who 

 reports that in the dog at least the parotid and the submaxillary may react 

 quite differently. When fistulas were made of the ducts of these glands it 

 was found that the submaxillary responded readily to a great number of 

 stimuli, such as the sight of food, chewing of meats, acids, etc. The parotid, 

 on the contrary, seemed to react only when dry food, dry powdered meat, or 

 bread was placed in the mouth. Dryness in this case seemed to be the 

 efficient stimulus. Since the How 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 oblongata ; its exact position 

 is not clearly defined, but possibly it is represented by the nuclei of origin of 



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

 1 Text-book of Physiology, edited by Scbafer, 1898. 



