As can be seen from a glance at a picture of the skeleton, the ribs are long, fiat, curved bones which are connected by joints with the vertebral column behind and in front with the sternum or breast bone. Only the upper seven ribs are articulated (or jointed) directly with the sternum, for the lowest two have their front ends free and are known as ‘floating ‘ribs, while the remaining three have their front ends attached to one another and to the sternum by long pieces of cartilage.
It is important to notice the direction of the ribs, which is downwards and forwards, for this gives the whole clue as to how they work. Taken as a whole the ribs, sternum and spine form a closed box which is called the thorax. The intervals between the ribs are filled with muscles which are capable of varying the capacity of the chest and thus make breathing possible. The interior of the thorax is filled with the heart and great vessels and with the lungs, which may be compared to a sponge filled with air. If the capacity of the chest is increased by the action of the muscles, the lungs must also expand and increase in size to fill the now greater thoracic cavity. In so doing they suck air in through the nose and down the windpipe or trachea.
It will be seen, therefore, that the essential thing in breathing is to make the capacity of the chest greater. This is done by the muscles which lie between the ribs, for their action is to raise the ribs and especially their front ends, together with the sternum. If the front ends of the ribs, which lie normally lower than their posterior ends, are raised, the distance from the front of the chest to the back is increased and so the size of the chest cavity is also increased. At the same time the muscles turn the lower edges of the ribs outwards so that again the capacity of the chest is increased.
The rising of the ribs is not the only way in which the capacity of the chest is increased, for there is a great sheet of muscle, called the diaphragm, which is fixed in a circle all the way round the lower part of the thorax and separates it from the abdomen. When this muscle contracts it pulls
itself downwards out of the thorax into the abdomen, thus increasing the depth of the chest from top to bottom. This is the whole mechanism of respiration reduced to its simplest terms.