Fig. 12. In the above, the shaded outlines indicate the shape of the bony cage of the chest during inspiration, and the lighter ones the same during expiration. The alterations in the position of the ribs and in the diameters of the chest, giving rise to its greater capacity during inspiration, are evident.
Attention will now be directed to some facts that it is of the utmost importance to clearly understand, if one is to know how to breathe and the reasons for the method employed. The lungs are contained in a cavity the walls of which are made up of a domed muscular (and tendinous) structure below, and elsewhere of bony and cartilaginous tissues filled in with soft structures, chiefly muscles. This cage is lined within by a smooth membrane which is kept constantly moist by its own secretion. The lungs are covered by a similar membrane, both of these fitting closely like the hand to a glove, so that there are two smooth membranes in opposition. It cannot be too well remembered that these two, the inner surface of the chest walls and the outer surface of the lungs, are in the closest contact. This is so whatever the changes that take place in the size and shape of the chest. The lungs are concave below, and so fit accurately to the fleshy partition between the chest and the abdomen which constitutes the lower boundary of the chest, if we may use the term "chest" somewhat loosely. Above, suiting the shape of the chest, the lungs are somewhat conical.
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.
Fig. 13. This figure is intended to indicate, in a purely diagrammatic way, by dotted lines, the position of the diaphragm (1) when inspiration is moderate, and (2) when very deep. The unbroken curved line above the dotted ones indicates the position of the diaphragm (only approximately, of course) after expiration.
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.