MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
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63. Serratus magnus, expiratory above, inspiratory below. 
I observed in the Dog, as in the Ass, the superior fibres of the serratus magnus to 
be expiratory, the inferior inspiratory, and those fibres in the centre neutral, § 51. 
64. In the Dog : serrati postici, superior 1 fibres inspiratory , inferior expiratory . 
The serratus posticus superior (fig. X. 10) was throughout inspiratory. The ser- 
ratus posticus inferior (fig. X. 11) was expiratory, its depressing action on the ribs 
overbalancing any action it might have to draw them backwards. The Seal (fig. VIII.) 
has a middle serratus posticus. See § 53. 
65. In the Rabbit, both serrati postici are inspiratory. 
In the Rabbit (fig. XI.) I observed that both serrati postici were throughout in- 
spiratory ; indeed their fibres are so amalgamated that they form one muscle. The 
diaphragmatic ribs of the Rabbit, which are far apart, are drawn far backwards on 
inspiration by the combined influence of the serratus posticus and the levatores 
costarum. 
66 . Expiratory muscles. 
I observed that the rectus abdominis, the external oblique, the transversalis, and 
the sacro-lumbalis, all acted on expiration. 
6 7 - V irieties in the rectus, external oblique, and depressor of first rib. 
The rectus (16) is inserted in the Otter (fig. XII.), fleshy into the first rib ; in the 
Dog (fig. X.) and Monkey it is inserted into the sternum, its whole length, by a ten- 
dinous aponeurosis, from which arises a fleshy depressor of the first rib (16 a.). In 
the Rabbit (fig. XI. 16 a.) this depressor arises from the sternum and passes in front 
of the rectus ; it does the same in the Seal (fig. VIII. 15), but in this animal it forms 
the first fibre of the external oblique (17)? which combines with fibres to the first 
(15), second (15 a.) and third (15 b .) costal cartilages, to form one vast external oblique 
inserted into all the ribs. 
68 . The triangulares sterni and the transversales combine to form one large muscle. 
The triangulares sterni (19) form in the Dog*, the Seal-f- and the Otter ^ one con- 
tinuous web with the transversales (20) ; in fact the triangulares sterni combine with 
the transversales to form one extensive muscular web, the vast constrictor of the chest 
and abdomen that arises from the posterior surface of the sternum, from the xyphoid 
cartilage and from the linea alba, and is inserted successively into the anterior extre- 
mities of all the ribs from the second to the lowest, which ribs it draws downwards 
and inwards during expiration to constrict the chest and abdomen. 
* Fig. XI. b. Archives of the Royal Society. f Fig. VIII. b. ibid. J Fig. XIII. b. ibid. 
