MR. SIBSON ON THE BLOW-HOLE OF THE PORPOISE. 
119 
gives off a slip that ascends in front of the neck of the sac, and passes over it to be 
inserted into the anterior wall of the outlet. 
When the posterior or frontal bellies of these muscles act on both sides, they will 
form a sling which will draw backwards the anterior wall of the breathing canal, 
and compress it against the posterior wall, thus closing the canal below the openings 
of the anterior sacs. They will, I conceive, at the same time contract the outlets of 
the sacs. 
The anterior heads, or those from the maxillary bone, will draw forward the sling 
formed by the combined muscles and open the breathing canal. 
These two muscles cross each other, so that in the same muscle the anterior belly 
is most superficial, while the posterior belly is deepest. 
The most superficial of these tendinous slings is blended with the tendinous struc- 
ture common to a very strong pair of muscles (24*). These muscles arise from the 
superior maxillary bones to the side of the anterior deep pouch, and ascend over the 
sides of that pouch to be inserted into the strong gristly tendon (fig. 1-24) at the top 
of it. The combined muscle forms a long arched musculo-tendinous bridge over 
the anterior deep sac, that, when it contracts, compresses the sac. The tendinous 
bridge just described passes backwards underneath the two tendons (fig. 1-22.23) 
that close the deep portion of the breathing canal ; it then turns upwards behind 
those tendons, and is reflected forwards over them to form the insertion of two other 
muscles (fig. 2-10) that are superficial to the muscles just described. The combined 
musculo-tendinous web formed by these two muscles and their common tendon, 
superficial to the muscles described above, arches over the anterior deep sac ; with 
those muscles it combines to contract the anterior deep sac. 
A distinct set of fibres (40*) arises from the side of the inlet of the osseous canal ; 
these fibres pass forwards and upwards over the neck of the anterior deep pouch, to 
be inserted into the central tendinous web covering that pouch, the outlet of which, 
this muscle, I conceive, contracts. 
Another muscle, (41*) just behind this, seems to have a similar function. 
The anterior deep pouch (2.) differs from the other pouches in having no superficial 
wrinkles, and in having a regular muscular coat; it seems indeed to be rather a pouch- 
like dilatation of the breathing canal, than one of the regular sacs. 
Each posterior deep pouch (fig. 1. 2-3) is situated behind the breathing canal, be- 
tween it and the frontal bone. Its opening is horizontal, and looks downwards, and 
is half-surrounded in front by a cartilage (fig. 1-21). 
Several fan-like, half-pouch formed muscles (16.17.18,19*), that arise from the 
frontal bone, contract and empty the posterior pouch. 
To contract the mouth of the pouch there are two muscles. One (fig. 1-20) springs 
from the side of the central protuberance of the frontal bone, and forms a tendon 
that passes forwards and inwards encircling the inner side and front of the outlet ; 
* See figures in the Archives. 
