lo Sept., 1908.] Elements of Ammal Physiology. 



527 



pulse. The pulse wave is not single in character. Instruments for re- 

 cording the pulse and frequently the finger alone can detect a second 

 beat just after the crest has passed. This is the dicrotic wave and is 

 produced by the rebound from the semilunar valve when this closes and 

 the ventricle passes into diastole. 



The Lymphatic System. 



The blood, as it passes through the capillaries, does not come into 

 actual contact with the cells of the various tissues. The capillary, it 

 has been said, has an actual wall. This wall however is so thin that it 

 allows some plasma to leak throuEfh forming lymph which bathes the cells. 



Fig. 53. Diagram of the principal group of lymphaiic vessels. (After Quain.) 



The lymph is contained in the loose connective tissue around the cells 

 and all the interchange of nutriment,, oxygen, waste products, &c., which 

 takes place between the cells and the blood must be carried out through 

 this medium. When the pressure in the capillaries rises, as occurs in 

 local obstruction of the veins or in local dilation of the arterioles, e.g., 

 inflammation, the amount of lymph increases, and may collect to such an 

 extent that the tissue becomes oedematous or dropsical. There is a very 

 slow circulation of this fluid for it can be shown to pass from the tissue 

 spaces into tiny thin walled vessels which run in clusters, join together, 

 and finally form an easily detected tube running towards the heart and 

 close to the spine. This is the thoracic duct and it empties its contents; 

 into a large vein near the heart. In their course the lymphatic vessels 



