MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
511 
29. The lungs in birds are imbedded in the spaces between the ribs ; they only 
have a free surface invested with pleura anteriorly. The diaphragm in the Fowl 
forms with this free surface of the lung an enclosed cavity. 
In Mammalia each lung is enclosed in a distinct pleural sac, the whole lung being 
free, save where the air-tubes and great vessels are attached. This cavity is com- 
pletely closed above, protected by ribs, muscles and fasciae. The first rib is more 
intimately connected than any other with the sternum ; in the majority of animals 
it either directly articulates with that bone, or is united to it by a short firm cartilage. 
It is in the expansion in every direction of the upper part of the chest, and the great 
range of descent of the whole diaphragm, the progressive lengthening of the ribs, 
the arched and comparatively mobile spinal column, and the usually small, jointed 
sternum, that the mechanism of breathing in the Mammalia chiefly differs from that 
in Birds ; for in Birds the upper part of the chest is not closed in, the diaphragm has 
but a limited range of motion, the spinal ribs are nearly of a length, the spinal 
column is stiff, and the sternum is in one large piece. 
30. The costal cartilages of Mammalia are analogous to the sternal ribs of Birds. 
In the Porpoise (Plate XXVI. fig. VII.) a series of bone-like stiff costal cartilages 
articulate by distinct joints with the sternum and the six superior vertebral ribs. The 
Sheep, the Cow (Plate XXVII. fig. IX.), and the Pig have costal cartilages that articu- 
late by joints with the ribs and the sternum. These cartilages are every way analogous 
to the sternal ribs of Birds ; they differ merely in this, they are made not of bone, 
but of cartilage. 
31. Dorsal arch ; the ribs are longest at the centre of the arch, and gradually shorten 
above and below. 
In all animals the lower part of the sternum is more distant from the vertebrae 
than the upper. The increase in length of the ribs and in depth of the chest takes 
place gradually from above downwards. The first rib is the shortest, the rest of the 
ribs increase gradually until about the sixth or seventh, which are the longest and are 
usually alike in length to each other and to the few following ribs ; the three or 
four lower ribs gradually shorten, the lowest of them being the shortest. The dorsal 
vertebrae form an arch, the curves of which correspond with the varying length of 
the ribs. The superior curve of the dorsal vertebrae coincides with the gradual 
lengthening, and the inferior curve with the gradual shortening of the corresponding 
ribs. 
32. Influence of the dorsal curves on the movements of the ribs. The upper ribs 
approach to, the lower recede from one another. 
The following diagrams (D 1, D 2) represent in an exaggerated mode, the curves in 
the dorsal spine, and the changes in position of the ribs when raised, as modified by 
