MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
535 
riorly, where they recede from each other; the rest of them are throughout expira- 
tory ; they depress the ribs, make the lower edge of each rib glide towards the ver- 
tebrae in relation to the upper edge of the rib below, and bring forwards the spinal 
column. 
At the upper and anterior part of the chest, where the ribs approach nearer to each 
other, an inspiratory shortening of the fibres of the internal intercostal overweighs 
the usual expiratory action (fig. XIV. b). There the external and internal intercostal 
muscles act simultaneously in a diagonal direction. (See § 48, last portion of it.) 
The four inferior external intercostals inserted into the diaphragmatic set of ribs, 
are expiratory between the anterior and lateral portions of the ribs, where they are 
so far apart from each other as to overweigh the inspiratory gliding motion of the 
ribs towards the vertebrae, figs. XIII. b., XIV. b. (§ 49). Posteriorly near the spine 
they are all, save the lowest, inspiratory. See diagram M, fig. XV. b. The fibres 
inserted into the ninth costal cartilage cause it, during inspiration, to glide down- 
wards and backwards on the cartilage above ; so that the anterior or intercartilagi- 
nous fibres of the eighth external intercostal are inspiratory, while the lateral fibres 
between the ribs are expiratory, and the posterior fibres are inspiratory (figs. XIII. b., 
XI V.b.,XV.b). 
The seventh external intercostal, that of the lowest of the intermediate set, has its 
anterior fibres slightly shortened on expiration; its posterior, on inspiration. The 
whole of the lower internal intercostals are throughout expiratory, whether between 
the ribs or the cartilages; they make each lower cartilage glide forwards and up- 
wards on the cartilage or rib above. 
86. Levatores costar urn. 
The levatores costarum, which are nearly allied, almost identical, with the external 
intercostals, are all inspiratory; they elevate the ribs posteriorly, and the inferior 
muscles draw backwards and deepen the posterior curves of the ribs. See diagram 13, 
§ 9. The superior levators are but insignificant ; the ribs on which they act, although 
they are raised considerably, have no backward movement. The six or seven lower 
levators are of considerable size ; the ribs on which they act, though they are raised 
but little, are drawn backwards considerably (fig. XV. a.b). See § 50. 
87- Serratus magnus, chiefly expiratory. See §§51. 63. 
The serratus magnus, and the serratus posticus inferior, are the only other muscles 
besides the external and internal intercostals, that have, in some portion of them, an 
inspiratory, in others, an expiratory action. In the Dog (§ 63) and Ass (§51) the 
superior fasciculi of the serratus magnus are expiratory ; the inferior are inspiratory ; 
and the central fibres are neutral. 
In Man the greater portion of the fasciculi of the serratus magnus act visibly in 
violent expiration. Indeed, from the direction of the superior fasciculi, they must be 
