MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
509 
20. Spinal ribs. 
All the spinal ribs, except the two first, which have free anterior extremities, arti- 
culate with sternal ribs. Those ribs that articulate with the vertebrae, and likewise 
with costal ribs, are of about equal length, and have spurs from their lower edges to 
give attachment and purchase to various muscles. The lowest ribs articulate with 
the pelvis. The pelvic ribs have no spurs, are longer than the purely vertebral ribs, 
and usually articulate with sternal ribs that are joined to each other near the sternum, 
with which they have a common articulation. 
Each purely vertebral rib has two articulations, one with the body, the other with 
the transverse process of the vertebra ; this last attachment allows the rib to be raised, 
but prevents that free range of forward motion that is permitted in the Snake. 
21. Sternal ribs. 
The superior sternal ribs are very short, many times shorter than the correspond- 
ing vertebral ribs ; they become gradually longer, the lowest equalling in length the 
corresponding vertebral ribs. 
22. I have watched the motions of the chest in the living fowl, and observed the 
action of a few of the muscles by vivisection. Figs. VI. a.b. Plate XXV. represents 
the Swan ; in fig. VI. a. the chest is contracted ; in fig. VI. b. expanded. I could not 
distend the lung owing to the air-sacs ; but I imitated inspiration exactly by drawing 
the sternum further from the vertebrae. It will be seen from these diagrams that the 
elevation of the ribs pushes forward the lower part of the sternum much further 
from the spine than the upper. This is owing to the lower sternal ribs being much 
longer, and having therefore a much greater range of motion than the upper. 
23. Inspiratory muscles of the spinal ribs. Scalenus (1), lev at ores cost arum (2), external 
inter costals (7). Plates XXIV. XXV. 
The scalenus (1) is prolonged from the first to the second rib; it corresponds ex- 
actly with the levatores costarum (2), which muscles both elevate the ribs and expand 
the chest. The inferior levators are smaller than the superior, the range of their 
action being more limited. 
The external intercostals (7) are throughout shortened in fig. VI. b. inspiration. A 
portion of the muscle, with fibres almost horizontal, arises from the spur, and shows 
beautifully the mode in which the muscle acts to draw the upper edge of the lower 
rib backwards, in relation to the lower edge of the upper rib ; in fig. VI. b. inspiration, 
the great shortening of these fibres is well seen. 
24. Inspiratory muscles of the sternal ribs; sterno-costal (9 a); sternal intercostals (9). 
The superior sternal ribs are raised by the sterno-costal muscle (fig. VI. 9 a.), 
which corresponds with the scalenus ; it arises from the upper part of the sternum. 
