504 
MR. S1BSON ON THE MECHANISM OF RESPIRATION. 
are much shorter in diagram 2, inspiration, than in diagram 1, expiration. It follows 
that the external intercostals are inspiratory. 
The rib is raised by the levator costse and by the external intercostal, and the 
upper edge of the lower rib is made to glide backwards in relation to the lower edge 
of the upper rib by the external intercostal, whose fibres, passing downwards and 
forwards, necessarily pull the lower rib upwards and backwards. 
8. The internal intercostals are expiratory. Diagram A 1 . 
The line 3, representing the internal intercostal, whose fibres pass obliquely up- 
wards and forwards, is much shorter in diagram A 1 , expiration, than in diagram A 2, 
inspiration. It follows that this muscle is expiratory. The internal intercostal is 
expiratory, it lowers the upper rib, and causes its lower edge to glide backwards in 
relation to the upper edge of the lower rib. 
The line 4.4 represents a muscle in the Snake that passes over one rib to be in- 
serted into the next rib but one above ; it has the same action as the internal inter- 
costals. 
9. On inspiration the ribs are drawn backwards posteriorly , outwards laterally, and move 
forwards anteriorly. Diagrams B 1.2. 
Besides the elevation of the rib during inspiration each rib is drawn backwards, 
so as to increase the area between the ribs of the two sides. 
The levator costce draws the rib backwards, B 2. 
Diagram B 1 . B 2. 
The ribs are drawn thus backwards by the levatores costarum, 3.3, the origin of 
which is behind the centre of motion of the rib at its articulation with the vertebra. 
The levator costse (3) is shorter in diagram B 2, inspiration, than in diagram B 1, ex- 
piration. 
The backward motion of the ribs during inspiration (B 2) counterbalances to a great 
extent the forward motion of their free ends, due to their inspiratory elevation, 
diagram A 2. § 6. 
It follows that during inspiration the vertebral and lateral portions of the rib move 
