MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
529 
examining the machinery of breathing in these creatures, we shall find little difficulty 
in investigating that machinery in Man. 
The various drawings (figs. XIII. a.b., XIV. a. b., XiV.a. b.*) are from the human 
subject. They are in pairs and represent the same subject, and the same view before 
and after the lungs were distended. 
The chest in Man is a cavity of ever-varying capacity, whose walls can be enlarged 
everywhere simultaneously. The domed roof rises, the floor descends, and the dia- 
meter between the walls is increased in every direction. 
79. Dorsal arch; the longest ribs are at the centre of the arch. Thoracic, diaphragmatic 
and intermediate sets of ribs. Plate XXIX. fig. XIV. a. b. 
In Man, as in all other Mammalia (see § 31), the lower part of the sternum is more 
distant from the spinal column than the upper; a gradual increase in the length of 
the ribs and the depth of the chest takes place from above downwards. The first rib 
is the shortest ; the ribs progressively lengthen to the sixth ; to it the seventh and 
eighth, which are the longest, are about equal ; and the four lowest ribs gradually 
shorten, the lowest being the shortest. 
Man, in common with the other Mammalia, has a dorsal arch (see diagram D 1 
and D 2, § 32), the curves of which correspond with the varying lengths of the ribs. 
The upper curve looks forwards and downwards, and coincides with the gradual 
lengthening of the five superior ribs which are connected with the sternum by 
distinct cartilages, and which encompass and expand the upper portion and body of 
the lungs, and are the ribs of thoracic inspiration (see § 34). The lower curve looks 
forwards and upwards, and is adapted to the progressive shortening of the four lower 
ribs which are tipped with floating cartilages, and which enlarge the lower portion of 
the chest simultaneously with the descent of the diaphragm, and form the diaphrag- 
matic set of ribs (see § 34.35). To the hollow of the arch, at its centre, are fitted the 
three longest ribs, the intermediate set, or sixth, seventh and eighth, that take part 
with the five superior ribs in thoracic, and the four inferior in diaphragmatic respira- 
tion. The cartilages of these three ribs are linked together, so that when the superior 
cartilage is raised, the rest necessarily follow. 
80. Changes in the situation of the ribs on inspiration. Plate XXIX. figs. XIII. a. b., 
XIV. a. b., XV. a.b. 
During inspiration the ribs approach to or recede from each other according to the 
part of the arch with which they articulate ; the four superior ribs approach each 
other anteriorly, and recede from each other posteriorly ; the fourth and fifth ribs, and 
the intermediate set move further apart to a moderate, the diaphragmatic set to a 
great extent. The upper edge of each of these ribs glides towards the vertebrae in 
relation to the lower edge of the rib above (§ 6.7), with the exception of the lowest 
rib, which is stationary. 
* Figs. XIX. a. b., XXI. a , b., XXII. a. b., XXIII. a. b., XXIV., XXV. in the Archives of the Royal Society. 
MDCCCXLVI. 3 Z 
