118 
MR. SIBSON ON THE BLOW-HOLE OF THE PORPOISE. 
opening- uppermost, and retain that opening above the surface of the water during 
sleep and during the act of copulation. 
The muscles that open and close the blow-hole, and that act upon the various 
sacs, form one of the most complicated yet most exquisitely adjusted pieces of 
machinery that either nature or art presents. 
The blow-hole is stretched open, in front, by two muscles (fig. 2-6) that act from 
the superior maxillary bones, to draw forward the cartilaginous mass in front of the 
outer opening ; behind, by muscular fibres (fig. 2-7) that act from the frontal bones 
on the posterior and lateral edges of the blow-hole. 
To close the blow-hole, two sets of muscular fibres (fig. 2-5) spring from the frontal 
bone, pass obliquely forwards and inwards, one to each side of the opening, and are 
inserted into the gristly mass (fig. 2-4) in front of the blow-hole. When they act, 
they draw this gristly mass forcibly backwards, and close the blow-hole by pressing 
its anterior and posterior walls together, and compressing them against the posterior 
pouches and the prominence at the centre of the frontal bone. 
A set of muscular fibres (fig. 2-8) arises from the circuit of a depression formed on 
the cranium to each side of the bony canals ; these fibres pass over the anterior 
lateral pouch, blending with the muscles that close and open the blow-hole. On 
removing this, a covering of muscular fibres (fig. 2-8 a) that cling round the sac is 
exposed ; these arise from the anterior lateral part of the superior maxillary bones, 
and pass upwards and backwards to form a muscular envelope for the pouch on its 
inner and posterior surface ; the fibres return forwards to be inserted into the outer 
and anterior walls of the blow-hole. 
A fan-like muscle (fig. 2-9) arises from the circuit of the depression to be inserted 
into the posterior and lateral walls of the blow-hole ; this assists in opening it by 
spreading apart its walls. 
On removing this fan-like muscle, a remarkable series of similar fan-like muscles 
(11.12.13.14.15*) is exposed. These lie one above the other, and arise by muscular 
fibres from the circuit of the depression ; the fibres of each converge to a tendinous 
aponeurosis thinner than paper. 
Four or five of these fan-like thin muscles may be removed by careful dissection, 
one after another ; they act to expand different parts of the outlet of the anterior 
lateral sac, to draw backwards and expand the posterior walls of the great respira- 
tory canal, and to compress the posterior deep sacs. 
On removing these, two remarkable muscles (fig. 1. Archives, fig. 7-22.23) are 
brought into view that have eaeh a double muscular origin, one from behind forwards 
from the frontal bone (22 a. 23 a*), meeting another from before backwards and inwards 
from the superior maxillary bone (22 23 b*). These two origins combine in each 
to form a tendon ; these tendons (22.23) pass forwards in front, and below the neck 
of the anterior pouch, where they unite with their fellows ; each of these tendons 
* These numbers refer to drawings that are in the Archives of the Royal Society. 
