CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 459 



observers have also obtained an increase of secretion upon stimula- 

 tion of the splanchnic nerves. 



All observers seem to agree that stimulation of the spinal bulb 

 causes a secretion or increases a secretion already going on, a 

 result which indicates the existence of some nervous mechanism. 

 It has also been observed that the central stimulation of various 

 nerves, or other nervous events, may arrest a secretion already 

 going on, a result which seems to indicate an inhibition of the- 

 bulbar mechanism at its centre. The subject still needs further 

 inquiry. 



We have seen, 227, that in the salivary glands the pressure 

 which may be exerted by the fluid in the ducts is very considerable, 

 exceeding it may be even the blood-pressure in the carotid artery. 

 In this respect the pancreas differs from the salivary glands. 

 When, in a rabbit, a cannula connected with a vertical tube or a 

 manometer is placed in the pancreatic duct, the column of fluid 

 does not rise above a height corresponding to a pressure of about 

 17 mm. of mercury. But at this pressure the gland becomes 

 oedematous on account of the juice secreted passing back through 

 the walls of the ducts and alveoli into the connective tissue ; a 

 much higher pressure is needed to render a salivary gland 

 oedematous ; and whether the low pressure observed in the pan- 

 creas is due to the ease with which oedema takes place or to the 

 actual secretion not being able to reach a higher pressure cannot 

 be stated with certainty. 



253. The Secretion of Bile. The act of secretion of bile by 

 the liver must not be confounded with the discharge of bile from 

 the bile-duct into the duodenum. When the acid contents of the 

 stomach are poured over the orifice of the biliary duct, a gush of 

 bile takes place. Indeed, stimulation of this region of the duo- 

 denum with a dilute acid at once calls forth a flow, though 

 alkaline fluids so applied have little or no effect. When no such 

 acid fluid is passing into the duodenum no bile is, under normal 

 circumstances, discharged into the intestine. The discharge is due 

 to a contraction of the muscular walls of the gall-bladder and 

 ducts, accompanied by a relaxation of the sphincter of the orifice ; 

 both acts are probably of a reflex nature, the efferent impulses 

 passing along the splanchnic nerves, but the details of the mechan- 

 ism have not been worked out. 



The secretion of bile on the other hand, as shewn by the 

 results of biliary fistulse, is continuous ; it appears never to cease. 

 When no food is taken the bile passes from the liver along the 

 hepatic and then back along the cystic duct (the flow being aided 

 probably by peristaltic contractions of the muscular fibres of the 

 duct) to the gall-bladder, where it is temporarily stored ; hence in 

 starving animals, when no discharge is excited by food, the gall- 

 bladder becomes greatly distended with bile. But the secretion, 

 though continuous, is riot uniform. The rate of secretion varies, 



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