94 THE HUMAN BODY. 



name (trachys, rough). Formerly it was confounded with 

 the arterial vessels, which were supposed also to be designed 

 to contain air. 



At the height of the third dorsal vertebra, the trachea 

 divides into two portions, which are called bronchi, which 

 on reaching the root of the lungs divide into numerous rami- 

 fications designated bronchia or bronchial tubes, and be- 

 coming more and more slender. Of the two principal 

 bronchi, the right is larger than the left, and the left is twice 

 the length of the right. They are both, as well as their rami- 

 fications, up to a certain limit, composed of fibrous membrane 

 and incomplete cartilaginous rings. When their diameter has 

 decreased to less than one-fiftieth of an inch, they no longer 

 have the cartilaginous rings, and the mucous membrane 

 cannot be separated from their walls. They continue to 

 subdivide, and terminate, as already stated, in the pulmonary 

 vesicles, the agglomeration of which in clusters forms the 

 lobules of the lung. 



Independently of the artery and pulmonary veins by which 

 the venous blood reaches the lung, and, transformed into 

 arterial blood, is returned to the heart that is, besides those 

 which serve for the sanguification and circulation the 

 bronchial veins and arteries carry the blood through the 

 lungs which is destined to nourish the organ itself, and it is 

 probable that the tissue proper of the lung uses for itself part 

 of the red blood formed in its cavities. Numerous lymphatic 

 vessels are also found in the lungs. The nerves which are 

 distributed through the lungs come from the pneumogastric 

 and the ganglion ic nervous system. 



Respiration. Respiration is a function by which the 

 oxygen of the air is introduced into the blood, and by which 

 part of the useless and hurtful materials are expelled, in a 

 gaseous form, from the organism. It is divided into two 

 parts : inspiration, during which the atmospheric air penetrates 

 the pulmonary cells; and expiration, which expels the air 

 which has been changed during its stay in the lungs. On 

 reaching the cells of the lungs, the blood is separated from 

 the air by their walls and those of the capillaries, which 

 ramify over them. However thin these membranes may be, 



