354 THE HUMAN BODY. 



under a pressure which will tend to make it flow to the ve- 

 nous termination of the thoracic or the right lymphatic duct. 

 (2) On account of the numerous valves in the lymphatic 

 vessels (which all only allow the lymph to flow past them to 

 larger trunks) any movement compressing a lymph-vessel will 

 cause an onward flow of its contents. The influence thus 

 exerted is very important. If a tube be put in a large lym- 

 phatic, say at the top of the leg of an animal, it will be seen 

 that the lymph only flows out very slowly while the animal 

 is quiet; but as soon as it moves the leg the flow is greatly 

 accelerated. (3) During each inspiration the pressure on the 

 thoracic duct is less than that in the lymphatics in parts of 

 the Body outside the thorax (see Chap. XXV). Accord- 

 ingly, at that time, lymph is pressed, or, in common phrase, 

 is "sucked," into the thoracic duct. During the succeeding 

 expiration the pressure on the thoracic duct becomes greater 

 again, and some of its contents are pressed out; but on 

 account of the valves of the vessels which unite to form, the 

 duct, they can only go towards the veins of the neck. 



During digestion, moreover, contractions of the villi press 

 on the lymph or chyle within them and force it on; and in 

 certain parts of the Body gravity, of course, aids the flow, 

 though it will impede it in others. 



The Ductless G-lands Spleen, Thyroid, Thymus, Pit- 

 uitary Body, Suprarenals. There are in the Body several 

 organs of such considerable size and so constantly present 

 in vertebrate animals that a priori they would seem to be of 

 functional importance. Until quite recently, however, the 

 functions of nearly all of them were quite problematical, al- 

 though it has long been known that pathological changes in 

 some of them were associated with grave conditions of general 

 disease. Even yet their physiology is very incompletely known. 



When we speak of a true gland we mean an organ that 

 forms some definite secretion which it pours out in a separate 

 form, but the organs we are about to consider have no secret- 

 ing recesses and no ducts: nevertheless some of them un- 

 doubtedly make, and pass into the lymph and blood, substances 

 of great importance to the healthy working of the Body. 

 Some true glands indeed do this, quite apart from the manu- 

 facture of what is usually spoken of as their secretion. Why 

 so large an organ as the liver should be set apart for the for- 

 mation of so comparatively unimportant a digestive fluid as 



