MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
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backwards and outwards, while the free end moves forward ; thus the space between 
the ribs is deepened and widened at the same time. 
During expiration the ribs are lowered, their free ends move downwards, inwards 
and backwards; while the lateral and vertebral portions move downwards, inwards 
and forwards ; thus the area between the ribs is narrowed and made less deep. 
10. Inspiratory muscles; scaleni (fig. I. 1); levatores costarum (fig. 1.6); external 
intercostals (fig. 1.7) ; fyc. 
The inspiratory muscles in the Snake are the scaleni (1), descending from the cervical 
vertebrae to the first ribs, and by continuation to the succeeding ribs. The continuous 
fibres are parallel to and blended with those of the external intercostal muscles. 
The external intercostal muscles (7) are shorter in fig. I. e , inspiration, than in fig. I.d, 
expiration. There are several other inspiratory muscles. Two of these (32.34) arise 
from the vertebra and are inserted into the rib, below and in front of the levator 
costae, another (33) passes from rib to rib near the vertebrae ; these are probably 
chiefly muscles of progression; they are all shortened when the lungs are expanded 
and the ribs are raised. 
11. Expiratory muscles ; depressors of the ribs (fig. I. 31); internal intercostals 
(fig. I. 21) (pass within one rib) ; transversalis (fig. I. 20). 
The expiratory muscles . — A series of narrow, ribbon-like muscles (31), peculiar 
to the serpent tribe, lies in front of the vertebral portions of the ribs; each of these 
muscles takes its rise from the body of the vertebra, ascends obliquely outwards and 
passes in front of four or five ribs to be inserted into the lower edge of the fifth or 
sixth rib above. These muscles are attached about half-way between the vertebrae 
and the free ends of the ribs, and are much shortened when the lungs are flaccid. 
They pull the ribs downwards, and narrow the chest between the opposite ribs by 
drawing them nearer to each other, and by bringing forward their vertebral portion. 
They antagonize the levatores costarum. 
A muscle (21) arises from the upper edge of the rib just above the insertion of the 
depressor of the rib, this passes obliquely upwards and outwards in front of one rib, 
to be inserted into the lower edge of the second rib above, near to its free end. This 
is shorter when the lung is flaccid than when it is distended, and is in fact a con- 
tinuation of the depressor of the rib, the rib being interposed ; it represents the internal 
intercostal. 
A transversalis muscle (20) forms a musculo-tendinous web over the whole belly; 
it is attached to the inner surface of the rib where the depressor costse is inserted, 
and acts with the depressor costae to narrow the area between the opposed ribs. 
There are cutaneous muscles that assist in expiration in front of the transversalis, 
which are connected with the ribs externally. 
3 u 
MDCCCXLVI. 
