530 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
expiratory*; the two or three lower fasciculi, from their direction, probably act in 
deep inspiration ; but I have not observed them to do so. 
88. Serratus posticus inferior , expiratory. See §§ 53. 64. 65. 
In the Ass (§53) the four lower fasciculi of the serratus posticus inferior are in- 
spiratory, the two upper expiratory, and the two intermediate neutral. In the Dog 
(§ 64) all the fasciculi are expiratory, while in the Rabbit (§65) they are all inspi- 
ratory. From the direction of the fibres, and from the fact that the fibres are shorter 
when the lungs are flaccid than when they are distended, I infer that in Man the 
serratus posticus inferior is expiratory. 
89. Inspiratory muscles , ordinary respiration. 
I have observed the following muscles to be inspiratory. In ordinary inspiration, 
the levator anguli scapulae draws the scapula upwards away from the ribs so as to 
allow them unembarrassed play. The serratus posticus superior (I have seen it act 
in the Dog and the Ass) draws upwards and outwards the superior ribs into which it 
is inserted. 
90. Deep or difficult inspiration. 
In deep or difficult inspiration, the superior fibres of the trapezius assist to elevate 
the scapula. 
The sternum is raised by the sterno-cleido-mastoid and by the sterno-hyoid and 
thyroid muscles ; the origins of the latter take their bearings from the inferior max- 
illary bone, through the active intermedium of the genio-hyoid and the anterior belly 
of the digastricus. 
The pectoralis minor and the lower fibres of the pectoralis major raise the ribs to 
which they are attached. 
91-94. Muscles of expiration. 
91. In addition to the intercostal muscles, the greater portion of the serratus 
magnus, and I believe the serratus posticus inferior, the following muscles act during 
expiration (I have seen them act in the Dog and Ass, §§ 51. 53. 54. 63.). 
92. Expiratory muscles acting on the scapulae. 
The latissimus dorsi . — I have seen it act in Man in difficult expiration, especially 
in coughing, and 1 have also noticed it act on expiration in the Dog; it never con- 
tracts under any circumstances in inspiration. The latissimus draws the scapula 
forcibly downwards on the ribs ; it is antagonist to the levator anguli scapulae. 
In coughing, the latissirni are assisted in their action on the scapulae by the pecto- 
ralis major and the lower fibres of the trapezius ; at the same time, while the serratus 
* See Quain’s Plates of the Muscles, PI. XIII. 
