THE ALTERATION OF THE BLOOD. 245 



ive process, and to prepare it for that function on which the hfe of 

 the animal is to depend; and also to prepare the intestines for the 

 discharge of their duty. When, after birth, it begins to be separated 

 from the parotid gland, it has no means of pouring its secretion into 

 the stomach, ancl it gradually dwindles away, and disappears. 

 ■» 



THE BRONCHIAL TUBES, 



The windpipe pursues its course down the neck, until it arrives at 

 the chest. It there somewhat alters its form, and becomes deeper 

 and narrower, in order to suit itself to the triangular opening through 

 which it is to pass. It enters the chest, and preserves the same 

 cartilaginous structure until it ariives at the base of the heart, where 

 it separates into two tubes, corresponding with the two divisions of 

 the lungs. These are called the bronchial tubes. They plunge 

 deep into th? substance of the lungs ; these presently subdivide ; and 

 the subdivision is continued in every direction, until branches of the 

 trachea penetrate every portion of the lungs. These are still air- 

 passages, and they are carrying on the air to its destination, for the 

 accomplishment of a vital purpose. The lungs of the ox afford the 

 most satisfactory elucidation of the manner in which these air-tubes 

 traverse that viscus. '1 hey can be followed until they almost elude 

 the unassisted sight, but the greater part of them can be evidently 

 traced into the lobuli, or little divisions of the substance of the lung, 

 which are so evident here. The minute structure of these lobuli has 

 never been demonstrated ; but we may safely imagine them to con- 

 sist of very small cells, in which the bronchial tubes terminate, and to 

 which the air is conveyed ; and that these cells are divided from 

 each other by exceedingly delicate membranes. 



THE ALTERATION OF THE BLOOD. 



The blood has already been described as sent from the right ven- 

 tricle of the heart into the lung, and the blood-vessels dividing and 

 subdividing until they have attained a state of extreme minuteness, 

 and then ramifying over the dehcate membrane of these cells. The 

 blood, however, is in a venous state ; it is no longer capable of sup- 

 porting life ; and it is forced through the lungs, in order that it 

 may be rendered once more arterial, and capable of supporting life 

 and all its functions. For this purpose, these minute veins spread 

 over the delicate membrane of the cells, and for this purpose also, the 

 air has been conveyed to these cells by the bronchial tubes. 



Now, the chemical, it may almost be said the vital, difference be- 

 tween venous and arterial blood is, that the venous is loaded with 

 carbon, and deficient in oxygen. It here comes, if not in absolute 

 contact with atmospheric air, yet so close as to be separated only 

 by a gossamer membrane, which offers little obstacle to the power of 



