182 PHYSIOLOGY OF ALIMENTATION. 



other glands during activity. Such a rise in temperature 

 means that exothermic reactions are taking place within 

 the gland. These exothermic reactions may be chemical, 

 and we find support for this idea in the well-known fact 

 that an active salivary gland consumes more oxygen and 

 gives off more carbon dioxide than a resting one. We are 

 dealing therefore with oxidations in the gland. But from 

 MATHEWS' experiments we know that lack of oxygen increases, 

 at least within certain limits, the secretions of the salivary 

 glands. Under such circumstances the heat would have to 

 come from intramolecular oxidations, in other words, from 

 the analysis of complex compounds into simpler ones. We 

 meet with less difficulty if we seek the source of at least 

 part of the heat that is set free in certain physical changes. 

 Of primary importance in this connection is the well-known 

 fact, which PAULI has studied with particular care, that col- 

 loids in absorbing water set heat free. The swelling of 

 mucin and other colloids found in saliva might well there- 

 fore be considered as sources of heat. MATHEWS' observa- 

 tions would also find an explanation, for an accumulation 

 of carbon dioxide or the various poisonous substances formed 

 in living tissues in the absence of oxygen all increase the 

 affinity of tissues for water. 



We have yet to mention the paralytic secretion of CLAUDE 

 BERNARD. When both the cranial and sympathetic nerve 

 fibres passing to a gland are cut, all secretion ceases. After a 

 few hours the secretion begins once more and continues for 

 days. Then, the secretion falls off again and finally ceases. 

 Whether the secretion is due to degeneration (and supposedly 

 stimulation) of the nerve and ceases when this is complete, 

 as is generally believed, or whether the atrophy which the 

 gland gradually undergoes is the real cause of the cessation, 

 is not yet settled. 



3. The Reflex Secretion of Saliva. Having determined 

 the nervous paths over which impulses may reach the salivary 

 glands and excite them to activity, we have to discover the 



