544 
MR. S1BS0N ON THE MECHANISM OF RESPIRATION. 
front ; and between their cartilages they are inspiratory (§§ 49. 60. 85) ; between the 
intermediate set of ribs they are for the most part slightly inspiratory between the 
ribs, and expiratory in front between the cartilages (figs. XIII. a.b., XIV. a.b., XV. a.b ). 
The internal intercostals of the thoracic ribs are expiratory behind and inspiratory 
in front, if the ribs approach there, and are inspiratory between their costal cartilages. 
Between the diaphragmatic and intermediate sets of ribs and between their cartilages, 
they are throughout expiratory (§§ 48. 49. 61. 85, figs. XIII.-XV.). The levatores 
costarum draw the posterior portion of the lower ribs backwards (§ 86, diagram B, 
fig. XV. a.b). 
In the Ass and Dog, the upper fasciculi of the serratus magnus are expiratory, the 
lower inspiratory, and the intermediate neutral (§§ 51. 63). In Man, the greater 
part of the fasciculi are expiratory (§ 87). 
In the Ass, the lower fibres of the serratus posticus inferior are inspiratory and 
the upper expiratory (§ 52); in the Dog, and I believe in Man, they are throughout 
expiratory (§ 88). 
The following muscles I have observed to be inspiratory , in addition to those 
named above : — 
On a moderate inspiration, § 89. — 
The levator anguli scapulae ; 
The serratus posticus superior. 
On a deep or difficult inspiration, 90, — 
The superior fibres of the trapezius ; 
The sterno-cleido mastoid ; 
The sterno-hyoid and thyroid, with the genio-hyoid and digastricus ; 
The pectoralis minor ; 
The pectoralis major, its lower fibres. 
The following muscles I have observed to be expiratory , in addition to those named 
above (§ 93. 94). 
The latissimus dorsi draws down the scapula in coughing ; the trapezius, its lower 
fibres, the pectoralis major and the rhomboidei (inferred) combine with the latissi- 
mus dorsi and the serratus magnus to draw the scapula forcibly downwards and for- 
wards on the ribs. The recti and the external and internal oblique draw downwards 
the sternum and the ribs ; the external oblique draw the posterior portion of the ribs 
forwards, the internal oblique their anterior portions backwards. 
The transversales and the triangulares sterni combine together to form one muscle, 
the constrictor of the chest and abdomen arising from the sternum, xyphoid cartilage 
and linea alba, and inserted into all the ribs but the first (§§ 68, 94) ; in the Por- 
poise it rises into the neck, and is inserted into all the ribs. § 76. 
The sacro-lumbalis and the longissimus dorsi. 
The expiratory muscles are, combinedly, more powerful than the inspiratory. 
