MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
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A set of muscles (21), as in the Snake, arise from the upper edge of one rib, and pass 
upwards over the inner surface of another, to be inserted into the rib but one above. 
An internal intercostal (21) arises from the upper edge of that portion of the rib into 
the lower edge of which the last muscle is inserted. This muscle is inserted into the 
anterior portion of the rib above. The Chameleon has peculiar to it a muscle to 
narrow the chest, by drawing the ribs inwards and nearer to those of the opposite side. 
This muscle (32) arises from the body of the vertebra anterior to the articulation of 
the rib, and is inserted into the inner surface of the rib near the vertebral head. 
A long external depressor of the ribs (longissimus dorsi, 30) arises from the pelvis, 
and is inserted by many tendons into the lower edges of the ribs. 
The chest is narrowed anteriorly by the transversales muscles (19.19) that arise 
from the anterior end of the vertebral ribs, and are inserted, the upper portion into 
the sternum, the rest into the costal cartilages ; they cause the cartilages to bend on 
the ribs in expiration. 
The ribs are lowered by an external oblique (17). 
BIRDS, §§ 17-27 (Plate XXIV. fig. III. a.b.c, Plate XXV. figs. IV. V. VI. a.b.). 
17- Birds have, in addition to spinal ribs, a sternum and sternal ribs. 
Birds have a complete apparatus of ribs hinging on the sternum, in addition to, 
and articulated with, those hinging on the vertebral column. The vertebrae and the 
vertebral ribs combine with the sternum and the sternal ribs to complete a circuit, 
forming, as it were, a cylinder or cavity enclosing and protecting the lungs, heart, 
and abdominal viscera. In such a manner do they combine and articulate that, by 
the movements of the ribs, the cavity can be enlarged on inspiration and lessened on 
expiration. The sternum bears the same relation to the sternal ribs that the ver- 
tebral column does to the vertebral ribs. These sternal ribs are long slender bones 
that articulate at one extremity with the sternum, at the other with the vertebral ribs. 
18. Diagram C 1.2. On inspiration, the sternal as well as the spinal ribs are raised, 
glide on each other 6, 7) and move farther apart. 
Diagram C 1 represents the vertebral and sternal ribs in the position they have in 
expiration ; diagram C 2, that they take up in inspiration. It is here, as in diagram 
A 1.2, inferred that the ribs, both sternal and vertebral, are all straight rods of equal 
length, hinging on the vertebrae V and sternum S with upward and downward 
motions. 
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