528 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
74. Special inspiratory muscles in the Porpoise, Plate XXVI. fig. VII. 27-8.8. 
There is no scalenus to any rib save the first; a muscle (27) passes from the cervical 
transverse process to three or four of the superior ribs that must act to raise them ; 
and muscular fibres (8.8) pass from the first rib just below the insertion of the sca- 
lenus, and from the second rib to be inserted into the third, fourth and fifth ribs ; these 
muscles are probably also inspiratory. 
75. The serrati postici are amalgamated, fig. VII. 10.11. 
The serrati postici are, in the Porpoise, amalgamated to form one muscle, or at least 
one tendinous web ; the fibres to the lower edge of the sixth rib mingle with those 
to the upper edge of the seventh ; the two fasciculi above are inserted into the upper 
edges of the ribs ; those below into the lower edges. 
. 76. Internal oblique . 
The internal oblique passes from rib edge to rib edge to be inserted into the an- 
terior ends of the seven lower ribs. 
The great caudal muscle is attached to the five lowest ribs. 
77- Power of the Porpoise to remain long under water accounted for. 
The true source of the power enjoyed by the Porpoise to remain so long under 
water is, I conceive, its capability of renewing almost the whole volume of air that is 
contained in the lungs at each inspiration, as the complete preceding expiration 
leaves scarcely any adulterated air in the lungs. 
The animal may descend under water with a far purer and far larger stock of air 
than land animals can, by any effort, obtain ; of course when the animal dives very 
deep the quantity of air in the chest cannot be great. 
MAN, §§ 78-100. Plate XXIX. figs. XIII. a. b., XIV. a. b., XV. a. b. 
78. All the creatures we have hitherto examined are in some measure illustrative 
of the mechanism of respiration in Man. In some point or other he partakes in 
structure with each of them. 
His three lowest, the floating ribs, resemble the simple vertebral ribs of the Snake. 
His sternum and costal cartilages are analogous to the sternum and spinal ribs of 
birds. In the possession of an intermediate set of three ribs, the sixth, seventh and 
eighth, whose cartilages are all linked together, and which partake of the functions of 
the thoracic set of five ribs above, and diaphragmatic set of four ribs below, he re- 
sembles the Calf, the Ass, the Sheep and the Pig. He has flexible and elastic costal 
cartilages fixed to the rib and hinging on the sternum, in common with the Seal, the 
Dog, and the Rabbit. He has a large portion of lung in the neck, like the Porpoise, 
subject to cervical respiration, and he possesses a clavicle and flat superior sternal 
bone, and other points that ally him with the Monkey. Having broken ground by 
