Organs of Respiration and Circulation. 



571 



tions of these important parts of the animal organism, it will be 

 necessary, first, to take a somewhat rapid glance at the processes of 

 digestion and assimilation, for the purpose of placing the office 

 of the lungs in a clearer view. The propriety of this course will 

 doubtless be admitted when we state that here are to be traced the 

 various changes which the nutritious part of the food passes 

 through, prior to entering the circulating fluid, the blood, to con- 

 tribute to the support of the frame. During life the repeated 

 demand for new materials to supply the constant waste of the 

 tissues, which arises from a multiplicity of causes, gives origin to 

 those sensations which are designated hunger and thirst. The 

 former of these shows the requirement of solid, and the latter of 

 fluid alimentary matters ; and they only yield to the proper 

 amount of food and drink being received. Notwithstanding that 

 both the quantity and the quality of the food which is partaken of, 

 will depend on the habits and conformation of each particular 

 animal, still in all it undergoes a successive and similar series of 

 changes before it ministers to the wants of the system. In the 

 mouth it is masticated, or divided by the operation of the teeth 

 into smaller masses, and while this reduction in size is being ac- 

 complished, it is mixed with the saliva, a fluid abundantly fur- 

 nished by the ducts of the contiguous glandular structures. This 

 insalivation of the food produces both a chemical and mechanical 

 effect ; by the former the mass is fitted for digestion by the alka- 

 line action of the saliva, and by the latter for deglutition by being 

 rendered soft or pulpy. Thus prepared, the food descends the 

 gullet and enters the stomach, where, uniting with the gastric 

 juice, it is subjected to a second chemical change, in which lactic 

 and hydrochloric acid are chiefly concerned. This process, com- 

 monly called the digestive, is effected, as we have seen, by the 

 succus gastricus ; a fluid which is secreted within the follicles of 

 the stomach, whence it is poured on the reception of alimentary 

 or other matters. 



Digestion thus converts the aliment into a chymous mass, and 

 portions of this are alternately passed out of the stomach into the 

 intestinal canal, where they are mingled with the hepatic and 

 pancreatic secretions furnished by the liver and pancreas. The 

 result of the commingling of these fluids with the chyme is the 

 speedy separation into its nutritious and non-nutritious parts, to 

 which is given the name of chylification ; and, like the other 

 changes we have described, this separation is most probably pro- 

 duced by chemical agency.* The chyle thus formed is next pre- 

 cipitated upon the villous coat of the intestines, to be absorbed or 



* For fuller particulars of these A'arious processes, see ' Lecture on the Digestive 

 Organs/ Society's Journal, vol. ix. part i. 



