MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
523 
tion is, as in the Pig (diagrams EF, § 3/), distributed in two directions. In one direc- 
tion (K 2 ), the vertebrae are thrust backwards by the rib, and the sternum forwards by 
the cartilage, the chest being deepened ; in the other direction (L 2 L 4 ), the outer por- 
tion of the rib and cartilage is thrust outward, the posterior portion of the rib is 
drawn backwards, and the anterior portion of the cartilage thrust forward. The 
greater and more extensive the play of breathing the longer are the costal cartilages, 
as we see in the Seal (fig. VIII.), and to a less degree in the Otter (fig. XII.). Both of 
these animals require very great play of respiration and unusual power to expel and 
renew the air at each respiration, owing to their diving habits. In the Dog, the Otter, 
the Rabbit and the Monkey, the first costal cartilage is stiff, and is firmly impacted in 
the sternum ; in the Seal, the first rib is long and flexible, and hinges moveably on 
the sternum. During inspiration in the Dog* all the ribs are drawn backwards, the 
superior ribs to a less extent than the inferior : the first, second and third ribs are 
stationary at the anterior extremity, but all the rest move backwards through their 
whole extent, though more posteriorly than anteriorly. 
5/. The thoracic and diaphragmatic sets of ribs*. 
The upper edge of each rib glides backwards in relation to the lower edge of that 
above, and at the same time the upper edges of those costal cartilages that hinge on 
the sternum have a reverse direction, and glide forwards and inwards in relation to 
the lower edges of the respective cartilages above. The superior ten cartilages 
articulate moveably with the sternum, the tenth imperfectly. The ten superior ribs 
act as ribs of thoracic expansion ; as the diaphragm ascends behind the sternum 
some distance into the chest the three lowest of them act with the eleventh, twelfth 
and thirteenth, or diaphragmatic set of ribs, to expand the chest when the diaphragm 
descends (see §§ 34. 35). 
The floating cartilages of the three lowest ribs glide in the same direction as the 
ribs on which they ride ; the upper edge of each cartilage sliding backwards in rela- 
tion to the lower edge of the cartilage above. The six superior ribs approach each 
other, especially anteriorly ; the seventh, eighth and ninth, are about neutral ; all the 
lower ribs recede from each other. 
58-69. Muscles acting on the Ribs. 
58. The scaleni of the superior Jive or six ribs are much developed. 
The scaleni are very greatly developed in all those animals with flexible cartilages. 
The scaleni of the first rib are very insignificant in the Seal and Otter, whose cervical 
vertebrae are very moveable ; while they are well-developed in the Rabbit and the 
Monkey, whose necks are less moveable. 
These animals all have long and powerful scaleni that pass over the first two ribs 
and are inserted into the ribs below them near their costal cartilages ; in the Dog 
* Fig. XI. c.d.e.f. Archives of the Royal Society. 
3 y 2 
