294 A MANUAL OF PHYSIOLOGY 



efficient afferent impulses may be set up, the gastric mucous 

 membrane, for example ; sulphate of zinc and sulphate of 

 copper act mainly in this way. Apomorphia, on the other 

 hand, stimulates the centre directly, and this is also the 

 mode in which vomiting is produced in certain diseases 

 of the medulla oblongata. The efferent nerves for the 

 diaphragm are the phrenics, for the abdominal muscles the 

 intercostals. The impulses which cause contraction of the 

 stomach pass along the vagi. Dilatation of the cardiac 

 orifice is brought about partly by the shortening of muscular 

 fibres, which spread out upon the stomach from the lower 

 end of the oesophagus, perhaps partly by nervous inhibition. 



jU II. The Chemical Phenomena of Digestion. 



The chemical changes wrought in the food as it passes 

 along the alimentary canal are due to the secretions of 

 various glands, which line its cavities, or pour their juices 

 into it through special ducts. These secretions owe their 

 power for the most part to substances present in them 

 in very small amount, but which, nevertheless, act with 

 extraordinary energy upon the various constituents of the 

 food, causing profound changes without being themselves 

 used up, or their digestive power affected. These marvellous 

 and as yet mysterious agents are the unformed or un- 

 organized ferments unorganized because, unlike some other 

 ferments, such as yeast, their action does not depend upon 

 the growth of living cells. Their chemical nature has not 

 been exactly made out ; some of them at least do not appear 

 to be proteids. But it is doubtful whether even one of the 

 ferments of the digestive juices has as yet been satisfactorily 

 isolated, and at present it is only by their effects that we 

 recognise them. Some of them act best in an alkaline, some 

 in an acid medium ; they all agree in having an 'optimum 1 

 temperature, which is more favourable to their action than 

 any other ; a low temperature suspends their activity, and 

 boiling abolishes it for ever. The action of all of them seems 

 to be hydrolytic ; i.e., it is accompanied with the taking up 

 of the elements of water by the substance acted upon. The 

 accumulation of the products of the action first checks and 

 then arrests it. 



