THE VASOMOTOR MECHANISM 383 



ing on into the neck, ends on the left side at the point already indi- 

 cated; receiving on its way the main stems from the left arm and 

 the left side of the head and neck. The thoracic duct, thus, brings 

 back to the blood much more lymph than the right lymphatic duct. 



Lymph-Nodes. At intervals along the course of various lym- 

 phatic vessels are structures consisting of cells so arranged as to 

 leave interspaces among them, through which interspaces the 

 lymph is forced to flow. These structures are the lymph-nodes 

 or lymph-glands and the peculiar tissue of which they are com- 

 posed is lymphoid or adenoid tissue. Lymph-nodes occur in the 

 neck, the groin, the axilla (arm pit) and in various other regions 

 of the Body. Certain structures in the wall of the small intestine 

 near its lower end, the so-called Peyer's Patches, are composed of 

 lymphoid tissue as are also the structures in the throat making 

 up the tonsillar ring. 



Functions of Lymph-Nodes. Two quite different functions 

 are attributed to the lymph-nodes. The first of these is that 

 previously mentioned (p. 304) of serving as the seat of lymph- 

 ocyte production. 



The lymph-nodes have also the additional function of filtering 

 the lymph that passes through them. This filtering action is 

 probably of great importance in confining micro-organisms to 

 the region which they first enter, since if they get into the lymph 

 stream they are arrested at the first lymph-node. It is thought 

 that the lymph-nodes are able also to arrest, for a time at least, 

 the spread of cancer-cells over the Body. The lymph-nodes located 

 on the channels draining the lungs become filled with dust that 

 has worked its way through the pulmonary walls into the lymph, 

 and that is prevented thus from spreading throughout the Body. 



Tonsils and Adenoids. The irregular ring of lymphoid tissue 

 surrounding the throat was referred to above. This at the front 

 shows two enlargements, one on each side, known as the tonsils. 

 At the back of the throat this same ring often in children becomes 

 enlarged by overgrowth until it obstructs the nasal passage and 

 interferes with the breathing. It may also obstruct the Eustachian 

 tubes and cause partial deafness. This overgrowth is known as ade- 

 noids. The removal of adenoids is a simple matter surgically, and 

 is advisable wherever there is evident obstruction of the breathing. 



The tonsils, which function in the manner of lymphoid tissue 



