MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
527 
ther in inspiration, they raise the sternum, the first ribs, and their costal cartilages. 
The sternum moves forward, the first ribs and cartilages move sideways, and the 
whole circuit is increased, the brim of the chest being at once widened and deepened. 
As the sternum and costal cartilages move forwards and outwards, they carry for- 
ward with them the sterno-hyoid and thyroid muscles ; and as the first ribs move 
outwards and backwards they carry the scaleni outwards and backwards, so that 
the same muscles that raise the sternum, first costal cartilages and first ribs, and in- 
crease the circuit they embrace, are in turn pushed forwards, outwards and backwards 
by the parts they raise, and the space for the lungs in the neck that they encircle is 
everywhere increased. 
72. The scaleni do not move the vertebrae laterally, though they draw them forwards 
and downwards in inspiration. 
In the Porpoise we have found that the scalenus is enormously developed although 
the cervical vertebrae are quite destitute of motion ; in the Seal, whose neck is re- 
markably flexible, the scaleni are quite insignificant (§ 58). It is very evident that 
the scaleni do not act to draw the vertebrae to either side ; how can they in the Por- 
poise? and in the Seal, whose neck is so bending, why are they not unusually deve- 
loped ? it is because they do not act on the vertebrae laterally, nnd because the 
mobility of the vertebrae would interfere with their action on the ribs. 
We find all through that the scaleni are small in those animals whose necks are 
mobile, and large in those that have inflexible necks. Though the scaleni do not 
move the vertebrae sideways, they do draw them downwards and forwards, at the same 
time that they raise the first ribs. The two fixed points in fact approach each other. 
73. The combined triangulares sterni and transversales form one vast constrictor of 
neck, chest and abdomen , which rises into the neck. 
The development of the scalenus, sterno-hyoid and thyroid, to cause the expansion 
of the lungs in the neck, is counterbalanced during expiration by the development 
in a remarkable manner of the sterno-costal, or triangulares sterni muscles (see § 68)*. 
In the Porpoise, the upper portion of these muscles, that in all other animals rises only 
to the second rib, ascends quite into the neck, embraces the lower half of the cervical 
portion of lung, and is inserted into the whole circuit of the first rib. In the Por- 
poise, as in the animals we formerly examined (§ 68), the sterno-costal muscles unite 
with the transversales ; and they unite to form an enormous muscle, one vast constrictor 
of the neck, chest, and abdomen. This combined muscle arises from the sternum and 
linea alba, and is inserted into all the ribs. With the aid of the other expiratory mus- 
cles, it constricts the neck, chest, and abdomen, and thrusts, through the interme- 
dium of the abdominal viscera, the large and bulging diaphragm almost quite up into 
the neck. 
* Fig. XVI. c. Archives of the Royal Society. 
