The distances of these bars are regulated by the position of the rigid body representing the spine. If all of them were moved upwards simultaneously, the first four would approxi mate, whilst all the rest would recede from each other. Therefore the positions of the different parts representing the spine in fig. 679. command and regulate these changes.
From this we learn, that the bars cannot rotate without changing their distances, and that when they are at 90° with reference to the body A a (fig. 677.), they are at their maximum distance from each other, and as they pass this position on either side, this dis tance diminishes.
In the human body the spine may represent the body to which the bars are attached ( fig. 677.). The movement of the ribs will obey the same law in receding or approaching each other, and whether they increase or diminish their intercostal spaces, will depend upon the relation they bear to the spine.
Fig. 682. is a cast, from a dissection of the thorax of a male subject, weight 1071bs., height 5ft.4in. This correctly represents the natural position of the ribs, when the thorax is in a state of complete expiration, or with only the residual air in the lungs. The position of the ribs is very oblique; the spine is curved ; therefore the relation of the ribs to the spine is different according to the curve, as are the bars to the body representing the spine (fig. 679.). It will be seen that the inferior six or seven ribs are at a more oblique angle to the spine than the superior ribs. The spine does not curve sufficiently to bring the upper ribs to an angle of 90° with the spine; there fore, if all the ribs were raised simultane ously, they would all increase the breadth of their intercostal spaces, whilst their sternal end would recede from the vertebra', and thus, by their elevation, the thoracic cavity would be enlarged, until they attained the angle of 90° to the spine. But if the elevation were carried beyond this point, the intercostal spaces would diminish, and thus the thoracic cavity would decrease. Fig. 684. is a pos terior view showing the sloping position of the ribs more clearly. Now, if the spine were perfectly erect, the ribs would have a greater range, and consequently the upper ribs could be elevated higher, and thus still more increase the thoracic cavity. A man
ribs are all elevated when the chest is inflated, and that the spine is straightened. Into the thorax (fig. 680. et seq.) we insufflated, or forced into the lungs 310 cubic inches of air, and a second cast was taken. The changed position of the ribs and spine is represented in figs. 681. 683. and 685. where it will be seen that all the ribs are raised ; their perpendicu lar distances or intercostal spaces are all increased, and the spine is more erect.
This experiment, therefore, demonstrates two things : 1st, that by artificially inflating the chest, the intercostal spaces are widened, and 2dly, that the spine becomes more erect. It is an experiment most unfavour able for showing these two points, because the altered shape of the thorax by insuifiation is not to be compared with the exceedingly enlarged condition produced by vital inspira tion ; in which case the spine becomes more erect, and the intercostal spaces consequently wider. By placing the fingers in the inter costal spaces of a living subject during deep inspiration and expiration, it may easily be perceived that in the former they widen, can expire a greater volume of air when perfectly erect, than in any other position. On the other hand, if we curve the spine, we limit the divergence of the ribs, because then we bring the ribs more into the po sition of a b and a' b' ( fig. 678.). Thus, in diseases of the spine, when angular cur vature is extensive, the ribs are materially limited in their capability of increasing the perpendicular depth of their intercostal spaces, and consequently the perpendicular depth of the thorax.
Fig. 666. is an instance of angular spine, reducing the ribs to their minimum distances without their moving. We have found by ex periment, that the greatest volume of air which persons with angular spine can expire, is little more than equal to the volume of air of an ordinary respiration ; i. e. from 20 to 40 cubic inches, instead of 180 to 200 cubic inches.