This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Muscles By Which The Ribs Are Movedare placed between them, forming two layers, sloped in opposite directions, and named the external and internal intercostal muscles.* In full inspiration muscles of the neck are also called into action to elevate the breast bone and first pair of ribs; and when breathing is difficult, particularly from asthma, additional force is obtained by fixing the arms, as, for example, by laying hold of an arm-chair; and these muscles extending between the chest and shoulders, being fixed at the latter attachment, exercise their action on the former. It is principally in inspiration that muscular force is called into requisition, expiration being largely aided by the elasticity of both ribs and lungs; for the ribs in inspiration are pulled forcibly out of their natural curve, and the elasticity of the pulmonary texture may be easily demonstrated by noting the rapidity with which the lungs of dead animals collapse after artificial inflation. In certain circumstances, however, a considerable exertion may be required in expiration, as, for instance, in playing a wind instrument or in glass-blowing; and then the abdominal muscles may be brought strongly into action both to push up the diaphragm and to depress the ribs.
* On the Continent it seems to be now generally admitted that both these sets of fibres are of service in inspiration; but in English text-books another theory continues to hold its ground, which is only thus referred to that the student may be warned against it as an error. The merits of the question cannot be here discussed.