120 
MR. SIBSON ON THE BLOW-HOLE OF THE PORPOISE. 
it splits into two tendons, one of which (20 a) is inserted into the outer side of the 
edge of the bony canal, and the other (20) into the partition of that canal. 
The other muscle (21*) arises close to the insertion of the last muscle, takes a 
sweep round the posterior and inner sides of the outlet of the pouch, and is inserted 
into the cartilage at the margin of that outlet in front. 
It appears that when the blow-hole is closed, the posterior pouches are compressed 
and emptied, while the anterior pouches are distended with air from the lungs. The 
anterior pouches will then cause the head and blow-hole to rise to the surface. When 
the blow-holes are open, the muscles that compress and empty the anterior sacs 
likewise expand the blow-hole ; it is clear that the anterior sacs that may be filled 
when the blow-hole is closed, will be emptied when it is open ; while the posterior 
sacs that are emptied when the outer passage is closed, may be filled when the 
passage is open. 
The anterior and posterior sacs alternate in their contraction and dilatation, the 
former being dilated and the latter contracted when the outer passage is closed, and 
the latter being dilated and the former contracted when the outer passage is open. 
Thus, first one then the other set of pouches may be dilated with air, to buoy up the 
head. 
The two passages (figs. 1.3-26.27) that descend through the cranium, are each 
surrounded by a muscular coat; they join below the septum (fig. 3-27), which may 
be regarded in the light of a vomer, and form an increasing cylindrical pouch, 
which is analogous to the upper part of the pharynx behind the soft palate. This 
pouch contracts suddenly, where it opens into the pharynx (fig. 3-34). Through the 
contracted outlet or communication of the pouch-like channel with the pharynx, the 
mouth of the larynx rises (35). This outlet is acted upon, and the mouth of the 
larynx is compressed and retained, by a sphincter (34). The whole tube is surrounded 
by oblique fibres (fig. 1-27) descending from before backwards. Exterior to this 
muscular investing coat, is a strong layer of muscular fibres (28) that arise from the 
superior maxillary bone, in front of the canal, and descend downwards and backwards 
until they turn the edge of the hard palate ; there they part into two sets ; one set 
(28 &) passes backwards strengthening the canal behind, another (28 a) descends a 
little forward to the os hyoides. 
The epiglottis (36) is long, firm and convex, and ends in an expanded lip. The 
arytenoid cartilages (37.35) are very large, strong and elongated, being seated imme- 
diately below, and contiguous to, the epiglottis. 
When the mouth of the larynx is opened, the long arytenoid cartilages are drawn 
downwards from the epiglottis and apart from each other by the crico- arytenoid 
muscles (39). The mouth of the larynx is closed by the action of the thyro-arytenoid 
(38) drawing the arytenoid cartilages over to the epiglottis. 
The remarkable apparatus described above forms the breathing passage of the 
* See figures in the Archives. 
