152 PHYSIOLOGY. 



These vessels are found within the body generally, even in those 

 structures which contain no blood-vessels, as the cornea of the eye. 

 The fluid within them always moves in one direction only: toward 

 the heart. These vessels, whose sources may be very different, unite 

 in their course to form larger vessels until, by continual union, they 

 terminate in two large trunks which empty into the subclavian veins 

 at their junction with the internal jugulars. The one emptying 

 into the left side is the thoracic duct, that into the right side is the 

 right lymphatic trunk. 



The large intestine possesses more lymphatics than the small, 

 so that richness of lymphatics in a given organ is not directly pro- 

 portionate to its absorbent functions. The number of lymphatics 

 has no constant relation to the elaboration of products secreted and 

 excreted by the glands, for they are numerous in the mammae and 

 liver, more scanty in the kidney, pancreas and thyroid, whilst they 

 are abundant in the center of the diaphragm. 



Structure of the Lymphatics. 



When the agriculturist wishes to drain his wet lowlands he 

 resorts to the use of pipes of great porosity. These are buried and 

 so arranged that the moisture of the soil very readily finds its way 

 into pipes, to flow along them and so be conveyed away. When the 

 arrangement of the pipes is suitable, the excess of water is carried 

 off. Should the drain-pipes become defective, or should their capac- 

 ity be less than that demanded of them, there at once results a stag- 

 nation with inundation of the land. For the water to find its way 

 from between the particles of earth and sand into the pipes it is 

 necessary that the latter be very porous and permeable a most 

 essential factor. 



The principle underlying the structure of the lymphatics is very 

 similar to that of the system of drain-pipes of the agriculturist 

 namely: porosity for the aim of each is to collect the excess of 

 their respective fluids and convey the same to certain desired 

 channels. 



The lymphatics drain off from the system of . the interstitial 

 spaces such substances, either foreign, or useless, or harmful to the 

 tissues, and deposit them in the lymphatic glands or carry them into 

 the blood to be rebuilt or excreted. 



This principle being kept in mind, the student can readily con- 

 ceive the nature of the lymphatics. 



They must be vessels of thin walls walls which allow of the 



