112 ELEMENTARY PHYSIOLOGY 



going on in the various tissues of the body, and also, during 

 youth, to allow of the growth and development of the system as 

 a whole. 



In the alimentary canal the various food-substances are simply 

 digested, that is, either converted into a soluble form, or else 

 separated into such minute particles that they are capable of being 

 transmitted through moist membranous structures. But now it is 

 necessary that the materials so digested enter the blood system. 

 They are then conveyed to the various tissues and added to their 

 structures, while the blood at the same time removes from these 

 tissues those materials which have performed their office, and 

 which are termed the waste products. The passage of the 

 nutritious substances from the digestive canal to the blood system 

 constitutes the process called absorption. 



We have noticed that the process of digestion commences 

 in the mouth, and is continued more or less throughout the 

 alimentary canal. We have also learnt that the whole of the canal 

 is lined with a soft mucous membrane which is richly supplied 

 with blood-vessels, and that as soon as the digestion of a food- 

 substance commences, the dissolved portions begin to penetrate 

 the soft membrane and pass through the thin walls of the minute 

 blood capillaries. Thus we become acquainted with one process 

 by which nutritious matter enters the blood a process which we 

 may term the absorption by blood-vessels. We shall presently 

 learn, however, that there is another and distinct process of 

 absorption, carried on by a complicated system of vessels 

 especially adapted for the purpose. 



The blood-vessels seem to exercise no choice in the materials 

 absorbed. Every kind of food is readily imbibed provided it is 

 dissolved or reduced to such a finely-divided state that it is 

 capable of permeating their walls. It must be remembered that 

 this absorption is carried on almost exclusively by the minutest 

 blood-vessels (the capillaries), the walls of which are exceedingly 

 thin. 



The other portion of the process of absorption referred to above 

 is carried on by a system of vessels called the lymphatic system 

 (see fig. 6), and the function performed by the vessels of this 

 system is of such an important character that they are often said 

 to constitute the absorptive system. 



The lymphatic system (Lat. lympha, clear water) is so called 

 because its vessels usually contain a watery fluid. It consists of 

 lymphatic capillaries, lymphatic vessels, glands, and two vessels 

 called lymphatic trunks, the larger of which is situated at the back 

 of the thorax and is called the thoracic duct. 



The lymphatic capillaries are very minute vessels (Lat. 



