LECTURE II. 



Delivered on June 2,2nd. 



THE CHEMICAL REFLEXES OF THE ALIMENTARY TRACT. 



MR. PRESIDENT AND GENTLEMEN, With the first appearance of 

 tnulticellular animals and the setting apart of a distinct layer of cells, the 

 hypoblast, for digestion and assimilation, the activity of the cells is at first 

 largely analogous to that of the previous unicellular organisms, each cell 

 of the alimentary epithelium seizing and ingesting the food particles 

 which reach the cavity of which the cells are a lining. The digestion is 

 therefore at first largely intracellular, and the ingestion of food particles 

 must be determined partly by chemical and partly by tactile or mechani- 

 cal stimuli arising in the food particles themselves. In the lowest of 

 metazoa, however, cells are found which exercise their influence on 

 external organisms by the production and excretion of poisonous material, 

 and a similar mechanism is soon evolved in the alimentary tract for the 

 production of digestive fluids. An elementary type of digestive apparatus 

 would therefore be a cavity lined with epithelial cells, which are stimulated 

 to the secretion of digestive juices by the chemical and mechanical 

 stimuli arising from food introduced into the cavity. The increase 

 in these digestive cells, necessitated by the greater bulk and greater 

 activity of the animal arising with the elevation in type, is procured in 

 all the higher animals by the formation of outgrowths of cells from the 

 lumen of the alimentary canal, outgrowths which give rise to the struc- 

 tures which we know as glands. The cells composing these glands are no 

 'onger subject to direct stimulation by any food present in the alimentary 

 canal. Each act of secretion of digestive juices must therefore involve 

 the transmission of a message of some sort or other from the lining 

 epithelium of the alimentary canal to the gland in question a transmis- 

 sion which is apparently analogous in all respects to the mechanism for 

 the production of movements in response to external stimulation. In the 

 higher animals all the motor reactions affecting skeletal muscles are 

 carried out by reflex actions and involve the cooperation of the central 

 nervous system. It is natural to suppose that the similar reactions, affect- 

 ing the chemical activity of glands, should be carried out in the same way, 

 and the earliest investigations of the influence of the nervous system on 

 glandular activity corroborated in all respects this supposition. The 

 taking of food into the mouth, for instance, is at once followed by a flow 

 of saliva, due to an induced activity of the various salivary glands. This 

 reaction was studied by Ludwig in the first half of the last century, and 



