524 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
their insertion extends to the eighth ; in the Otter and Monkey to the sixth ; and in 
the Seal and Rabbit to the fifth ribs. The scaleni act during the whole time of in- 
spiration to pull upwards the ribs into which they are inserted. 
59. Ster no-mast old, 8fc. 
The sterno-mastoid, sterno- hyoid and thyroid muscles, when they act from above, 
raise the sternum ; when the sterno-hvoid and thyroid so act, their origins are fixed 
by the genio-, and stylo-hyoid and the hyo-thvroid. 
60. External intercostal muscles. 
The actions of the external and internal intercostals are in principle the same as 
those of the Ass and Sheep, varying as the ribs’ movements vary (see §§ 46. 48. 49). 
The first seven external intercostal muscles, where they pass from rib to rib, are 
inspiratory ; the eighth (perhaps the ninth) are neutral anteriorly, and are inspiratory 
posteriorly. 
The fibres from the third, fourth and fifth ribs inserted into the costal cartilages 
are neutral, but those inserted into the seventh, eighth, ninth and tenth cartilages, 
which in the latter pass from cartilage to cartilage, are all strongly expiratory ; 
they bend the cartilage on the rib, and cause the upper edge of the lower cartilage to 
glide forwards in relation to the lower edge of the upper; movements the reverse of in- 
spiratory. The three lowest external intercostals are inspiratory posteriorly, expiratory 
in the middle, where they act on the anterior portion of the ribs, and inspiratory 
anteriorly, where they pass from cartilage to cartilage ; the anterior fibres cause the 
cartilage they are inserted into to slide downwards, outwards and backwards in re- 
lation to the cartilage from which they rise. 
6 1 . Internal intercostal muscles. 
The first internal intercostal between the rib is inspiratory on its anterior four- 
fifths, the second and third in their anterior half and third ; posteriorly they are ex- 
piratory ; all the lower muscles between the ribs are expiratory ; the expiratory action 
being less extensive in the superior than the inferior ribs, and in their anterior than 
their posterior portion. The fibres inserted into the ten superior cartilages are inspi- 
ratory, having the same relation to these cartilages that the external intercostals have 
to the ribs they are fixed to. 
The internal intercostals between the three ? lowest cartilages are expiratory. 
62. Elevator of the first costal cartilage in the Seal, Plate XXVI. fig. VIII. 10 a. 
In the Seal, whose flexible first costal cartilage hinges on the sternum, a muscle 
arises from the sternum and is inserted into the first cartilage that is analogous with 
the internal intercostal fibres between the cartilages, and which, like them, is inspi- 
ratory. 
