164 ARTERIAL AND VENOUS BLOOD. 



CHAPTER VII. 



Arterial and venous Blopd Nature of Respiration Structure 

 of the Lungs Conditions required for healthy Respiration- 

 Sound original Constitution Influence of hereditary Predis- 

 position Of wholesome Food, and good Digestion Of the 

 free Expansion of the Lungs Of Exercise of the Muscles and 

 Voice Of Cheerfulness and Depression of Mind Of pure 

 Air and Ventilation Examples of the bad Effects of vitiated 

 Air Respiration the Source of Animal Heat Causes of de- 

 ficient Generation of Heat Removal of such Causes Direct 

 and indirect Exercise of the Lungs Beneficial Effects of, 

 and Rules for Exercise Precautions to be observed in Dis- 

 eases of the Lungs, and in Persons predisposed to Con- 

 sumption. 



WE come next to treat of the lungs and of the 

 function of RESPIRATION ; but, in order to be clearly 

 understood, I must premise a few observations on 

 the circulation of the blood. 



The blood circulating through the body is of two 

 different kinds ; the one red or arterial, and the 

 other dark or venous blood. The former alone is 

 capable of affording nourishment and of supporting 

 life. It is distributed from the left side of the heart 

 all over the body, by means of a great artery or 

 blood-vessel called the aorta, which subdivides in its 

 course, and ultimately terminates in myriads of very 

 minute ramifications, closely interwoven with, and 

 in reality constituting a large portion of, the texture 

 of every living part. On reaching this extreme point 

 of its course, the blood passes into equally minute 

 ramifications of the veins, which, in their turn, 

 gradually coalesce and form larger and larger trunks, 

 till they at last terminate in two large veins, by 

 which the whole current of the venous blood is 

 brought back in a direction contrary to that in the 



