MECHANISM OF THE CIRCULATION. Ill 



The lymphatic vessels have a very irregular ^course, and 

 exhibit numerous swellings which are owing to valves. They 

 exist in every part of the body, and transport the chyle and the 

 lymph, drawn by their microscopical roots from the surface of 

 the mucous membrame of the intestines, or from the tissues 

 of the organs. They accompany the blood-vessels in their 

 course, especially the veins, and they are also found in great 

 numbers at the surface of the body in regions abounding in 

 subcutaneous veins, as in the limbs, face, and neck. They 

 are very numerous in the mesentery and around the intes- 

 tines; they unite in two principal trunks or reservoirs, one 

 of which is the thoracic duct, which rises through the chest at 

 the left of the spine, and opens into the left subclavian vein, 

 the other is called the great right lymphatic vessel (ductus 

 lymphaticus dexter), which runs parallel with the first, and 

 opens into the right subclavian vein. 



Mechanism of the circulation. Galen was the first to dis- 

 cover that the arteries contained blood and that they com- 

 municated with the veins, but he went no farther than this. 

 In 1553 Michael Servetus, guessing, so to speak, the pheno- 

 mena of the pulmonary circulation, indicated very exactly 

 the course of the blood and its elaboration in the lungs by 

 contact with the air. But the doctrine of Servetus rested 

 neither upon proof nor experiment, it resulted from a sort of 

 intuitive perception of the facts, and he was not aware either 

 of the impulsive force of the heart or the action of its valves. 



Other physiologists, like Servetus, had glimpses of the 

 truth and added new discoveries to his ; in fact, most of the 

 phenomena of the circulation had been suspected or indi- 

 cated at the commencement of the seventeenth century, but 

 all the knowledge on the subject was but a chaos of facts 

 and reasonings without unity, and often contradictory. It 

 required the genius of Harvey to extract from this chaos a 

 simple and irrefutably demonstrated system. 



Movements and sounds of the heart. The heart, the prin- 

 cipal agent of the circulation's the source of movements which 

 are not under the control of the will, but which are constantly 

 influenced by moral impressions and by sensations. These 

 movements consist in the alternate contraction and relaxa- 



