332 A MANUAL OF PHYSIOLOGY 



conclusion that each membrane becomes accustomed, and, so 

 to speak, ' immune,' to the secretion normally in contact 

 with it, although not necessarily to other secretions. It is 

 easy to multiply illustrations of this principle. 



Few tissues but the lining of the urinary tract or of the 

 large intestine could bear the constant contact of urine or 

 faeces. When urine is extravasated under the skin, or the 

 contents of the alimentary canal burst into the peritoneal 

 cavity, they come into contact with tissues which, although 

 alive, are much less fitted to resist them than the surfaces 

 by which they are normally enclosed ; and the consequences 

 are often disastrous. Leucocytes thrive in the blood, but 

 perish in urine ; blood does not harm the endothelial cells 

 of the vessels, but kills a muscle whose cross-section is dipped 

 into it. The defensive, or in some cases offensive, liquids 

 secreted by many animals are harmless to the tissues which 

 produce and enclose them : a caterpillar investigated by 

 Poulton secretes a liquid so rich in formic acid, that the 

 mere contact of it would kill most cells. The so-called 

 saliva of Octopus macropus contains a substance fatal to the 

 crabs and other animals on which it preys. The blood of 

 the viper contains an active principle similar to that secreted 

 by its poison-glands, but its tissues are not affected by this 

 substance, so deadly to other animals. 



The Influence of the Nervous System on the Digestive Glands. 



The greater part of our knowledge of this subject has been 

 gained by the study of the salivary glands, and especially 

 the submaxillary and sublingual, which lie superficially and 

 are easily exposed. 



(i, The Influence of Nerves on the Salivary Glands. All the 

 salivary glands have a double nerve - supply, from the 

 medulla oblongata through some of the cranial nerves, and 

 from the spinal cord through the cervical sympathetic 

 (Fig. 107). 



In the dog the chorda tympani branch of the facial nerve carries 

 the cranial supply of the sublingual and submaxillary glands. It 

 joins the lingual branch of the fifth nerve, runs in company with it 

 for a little way, and then, breaking off, after giving some fibres to the 

 lingual, passes, as the chorda tympani proper, along Wharton's duct 

 to the submaxillary gland. In the hilus of this gland most of its 



