The pressure of the air tends of itself to expand the lungs, which are highly elastic, even when one does not breathe at all. But if more air is to enter there must be additional space provided; hence greater expansion of the lungs can only follow an enlargement of the chest cavity in one or in all directions. These are spoken of as _diameters_. It follows that it is possible to conceive of the chest being enlarged in three, and only three, directions; so that it may be increased in size in its vertical, its transverse, and its antero-posterior diameter, or diameter from before backwards. This expansion, as in the case of all other movements, can be effected only by muscles, or, to speak more accurately, by neuro-muscular mechanisms. Exactly what muscles are employed may be learned from the accompanying illustrations and by observation. While it is highly important to know in a general way which muscles are chiefly concerned, or, rather, where they are situated, it cannot be deemed essential for every reader to learn their names, attachments, etc., down to the minutest details, as in the case of a student of anatomy proper. The author does, however, deem it of the highest importance that the student should learn by actual observation on his own person that his chest does expand in each of the three directions indicated above. It is not necessary to dissect to observe muscles; in fact, they can be seen in action only on the living subject. All who would really understand breathing should study the chest when divested of all clothing and before a sufficiently large mirror. He may then observe the following during a fairly deep inspiration: 1. The chest is enlarged as a whole. 2. The abdominal walls move outward. 3. The ribs pass from a more oblique to a less oblique position, and may become almost horizontal; their upper edges are also turned out slightly, though this is not so easy to observe. 4. Again, in the case of a very deep and sudden inspiration, the abdomen and the lower ribs also are drawn inward. The changes above referred to are brought about in this way: 1. The total enlargement is due to the action of many muscles which function in harmony with each other. 2. The chief changes are brought about by those muscles attached between the ribs (_intercostales_); but these act more efficiently owing to the cooeperation of other muscles which steady the ribs and chest generally, such as those attached to the shoulder-bones and the upper ribs; indeed, the most powerful inspiration possible can only be effected when most of the other muscles of the body are brought into action. One may observe that even the arms and legs are called into requisition when a tenor sings his highest tone as forcibly as possible, though this is often overdone in a way to be condemned. Art should not be reduced to a gymnastic feat.