CHAPTER XXII 



MECHANISM OF SECRETION 



THE process of secretion may be studied in the salivary glands, the pancreas, the 

 kidneys, and the mamma. The influence of nerves upon secretion is illustrated 

 by the salivary glands, that of hormones by the pancreas and mamma : the 

 secretion of the kidneys is also dependent partly upon chemical agents, but 

 largely upon the blood-pressure and blood-flow through the organ. 



The submaxillary gland. A dog, having been anaesthetised, is fixed on its 

 back and a vein cannula inserted into a saphenous vein. An incision is then 

 made on one side through the skin and fascia below the mouth extending from 

 the chin backwards for three or four inches nearly parallel with the line of the 

 lower jaw. At the posterior part of the wound the submaxillary gland may be 

 observed. The anterior belly of the digastric, which comes into view and con- 

 ceals the hilum of the gland, is drawn over laterally by a weighted hook: or it 

 may be cut away. Any oozing from small vessels is controlled by rinsing the 

 wound with dilute extract of suprarenal : larger vessels are tied. A muscle 

 the geniohyoid is now exposed : it is composed of transverse fibres. When it 

 is cautiously cut through, the duct of the submaxillary gland (Wharton's duct) 

 is seen passing obliquely forwards towards the floor of the mouth. It is accom- 

 panied by a smaller duct, that of the sublingual. Crossing these ducts is a 

 conspicuous nerve the lingual branch of the 5th. If this is drawn towards 

 the middle line with a blunt hook, it is seen that just before it crosses the ducts 

 it gives off a small nerve the chorda tympani which runs sharply backwards 

 and enters the hilum of the submaxillary gland, where the duct is emerging. 

 Tie a thread around the lingual above the place where the chorda leaves it, 

 and, without injuring it, clear a short length of the chorda so that a small pair 

 of flat electrodes can be placed underneath it. Stimulate by induction shocks : 

 the duct will fill with saliva. 



Place a wet thread round the duct and slip a pointed piece of paraffined paper 

 under it. Make a snip into it with fine scissors ; pass a " finder " into the aper- 

 ture. Substitute for the " finder " a very fine metal or glass cannula, and tie 

 this in : a piece of small rubber tubing can be used to conduct the secretion 

 beyond the edge of the jaw, where it can be allowed to drop into a beaker. 



On the same side of the neck make a longitudinal incision through the skin 

 and fascia, and separate the muscles so as to expose the carotid artery and the 

 common trunk of the vagus and sympathetic nerves. Ligature this combined 

 nerve low down, and place the upper end on a second pair of electrodes. The 

 two pairs of electrodes for chorda and sympathetic respectively are con- 

 nected to a commutator without cross-wires, and this with the secondary coil, 

 so that stimulation can be led into either pair at will. 



1. Stimulate the sympathetic. A few drops of thick viscid saliva are 

 secreted. 



2. Stimulate the chorda. There is a rapid flow of watery saliva lasting aa 

 long as the excitation is continued. 



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