MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
513 
and eighth libs are linked anteriorly to a common cartilage ; their vertebral articu- 
lations occupy the hollow of the arch ; when they are raised they retain their original 
distance. The six ribs below these have floating cartilages, hinge with the lower 
curve forward of the vertebrae, either immediately below or a little in front of each 
other, and they, when raised, become further apart. 
In the Pig* and the Sheep-j~> whose short and firm costal cartilages articulate by 
joints both with the sternum and the ribs, the superior six (in the Sheep seven) ribs 
gradually lengthen ; they hinge on the superior curve of the dorsal arch, and when 
raised they come nearer to each other ; the two next ribs have united cartilages, and 
are neutral ; while the lower ribs during inspiration become more remote. 
In the Dog* whose costal cartilages are long and flexible, permitting free thoracic 
play, during inspiration the superior six ribs approach each other ; the next two are 
neutral, and the remainder are more distant. 
Man too partakes in this arrangement. Plate VII. figs. XIII. a. b., XIV. a. b., 
XV. a. b. 
34. Three sets of ribs. The superior, or thoracic ; the inferior, or diaphragmatic ; 
and the intermediate. 
The longest ribs which hinge on the hollow of the dorsal arch, and which neither 
approach to, nor recede from, each other during inspiration, form as it were a neutral 
ground between the superior ribs that approach to, and the inferior that recede from 
each other. The superior ribs, with their costal cartilages, form a complete circuit 
with the vertebrae and sternum to enclose the lungs and heart ; it is their motion, 
increasing the area within them, that constitutes true thoracic inspiration ; these 
form the thoracic set of ribs. The inferior ribs, each tipped with a floating cartilage, 
flank the abdomen on each side, have between them an open space in front, and give 
origin to the diaphragm ; these form the diaphragmatic set of ribs. Their action is 
in aid of diaphragmatic respiration, their office being to enlarge the area of the chest 
simultaneously with the descent of the diaphragm, without which descent they would 
act, not on the lungs, which unless drawn down by the diaphragm are above them, 
but on the abdominal viscera, the more important of which it is their duty to protect. 
The intermediate set of ribs share duties with each of these sets, forming with the 
superior set thoracic, with the inferior, diaphragmatic ribs. 
35. The proportion of the thoracic to the diaphragmatic ribs depends on the proportion 
of the upper lobes of the lungs to the lower. 
The relative proportion that the sterno-vertebral ribs, or those of thoracic respira- 
tion, bear to those of diaphragmatic respiration, depends on the size and form of the 
upper lobes of the lungs, compared with the lower. 
In the Ass, the upper lobes are small and narrow, the lower are large and full at 
their posterior part. The lungs are short in front at the sternum, long and broad 
* Figs. VII. a. b. Archives of the Royal Society. f Fig. VIII. a. b. ibid. J Fig. XI. a.b.c.d. ibid. 
MDCCCXLVI. 3 X 
