520 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
though more posteriorly than anteriorly; the two or three lowest ribs do not glide on 
each other. The lower external intercostals are almost entirely expiratory, as in 
them the gliding action and the separation of the ribs in some measure counter- 
balance each other, the fibres of the muscle being either expiratory or inspiratory, as 
the one action or the other overbalances. In the Sheep*, the three lowest muscles 
are divided into two portions, the anterior being expiratory, the posterior inspiratory. 
The fibres of the external intercostals between the floating cartilages contract 
during inspiration, causing the upper edge of one cartilage to glide backwards on the 
lower edge of that above ; we have thus a curious distribution of opposite functions in 
the same muscle, the anterior fibres, those between the cartilages, being inspiratory, 
the middle, between the fore-part of the ribs, expiratory, and the posterior, inspi- 
ratory. The whole of the internal intercostal muscles are expiratory, both those 
fibres inserted into the cartilages and those into the ribs. We have then a complete 
transposition of functions in many parts of these muscles; the external intercostals 
between the ribs being inspiratory above, expiratory below, those parts between 
the cartilages being expiratory above and inspiratory below. 
The muscles between the neutral or intermediate set of ribs are almost neutral in 
action. 
50. Levator es cost arum. 
The whole of the levatores costarum are inspiratory ; they draw upwards and thrust 
outwards the angle of the rib, elevate the ribs behind and pull backwards the pos- 
terior part of the diaphragmatic (diagram B. § 9), the intermediate, and a portion of 
the thoracic sets of ribs. They assist the external intercostal in making the upper 
edge of one rib glide backwards on the rib above (§ 6. 7), and in so thrusting back- 
wards the vertebrae with which the rib articulates. 
^ 51-54. Certain muscular actions of the Ass observed on vivisection. 
51. In the Ass the fibres of the serratus magnus are expiratory above, inspiratory 
below, neutral in the centre. 
In the Ass, I observed, on vivisection, that during inspiration the two lower fibres of 
the serratus magnus were shortened, acting on the ribs of the intermediate set, which 
were drawn backwards in their whole extent; the origins at the scapula of the two 
lowest fasciculi of the serratus magnus are above the articulation with the vertebrae 
of the two ribs on which they act. The sixth fasciculus of the serratus to the sixth 
rib was neutral, neither expiratory nor inspiratory. That to the fifth shortened very 
slightly on expiration; all the superior fibres acting on the first, second, third and 
fourth ribs acted very decidedly to draw down those ribs on expiration. The articu- 
lations of these ribs with the vertebrae are considerably above the attachment of 
the serratus to the scapula ; the centre of action of the serratus being considerably 
* Fig. VIII. Archives of the Royal Society. 
