15. The lungs, through which all the blood in the body passes, receive likewise the air which we draw in every time we breathe. They consist of two large bodies called _lobes_; from their situation called the right and left. The air is conveyed into them by the wind-pipe, called also the _trachea_, and the _aspera arteria_. On entering the cavity of the breast, the wind-pipe divides into two large branches called the _bronchiæ_; one of which goes to the right and the other to the left lobe of the lungs. By the further division and subdivision of these vessels the lungs are filled with an innumerable multitude of little tubes, terminating in exceedingly minute bladders or cells, which are the final receptacles of the air sucked in when we breathe. Each of these cells is surrounded with a kind of network of blood-vessels exceedingly small, and consisting of very thin membranes; so that, in passing through the lungs, the blood is exposed as much as possible to the action of the air. 16. It is a matter of great importance to find out what is the use of this exposure of the blood; and great disputes have taken place concerning it. In former times it was supposed that the blood received from the air a _vital spirit_, without which it would have been totally incapable of performing its offices in the body. Later physiologists endeavoured to explode this notion. Dr. Hales particularly, by shewing that the circulation of the blood through the lungs might be continued by inflating and contracting them alternately by the fumes of burning brimstone, endeavoured to prove that the use of the air was only to give the lungs an opportunity of dilating and contracting alternately, by which means principally he thought the circulation might be carried on. This continued to be the most common hypothesis as late as the time of Dr. Huxham. It was however thought also that by the compression of the air the blood was altered in its texture, its bulk, &c. Accordingly Dr. Huxham tells us in the preface to his treatise on air and epidemic diseases, that “air fit for respiration ought neither to be too hot, nor very cold; for the use of the inspired air is to temperate the blood, which would otherwise grow too hot, and putrefy, as is evident from the experiment of the most excellent Boerhaave made in a hot house; for, if the air is more hot, or even equally hot, as the blood of any animal, it certainly soon dies.”[47] [47] This certainly does not hold good if we suppose the heat of the atmosphere to be indicated by a thermometer; for we are assured that animals can live in a heat much superior to that which raises the mercury to 97.