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VII. On the Blow-hole of the Porpoise. By Francis Sibson, Esq. 
Communicated hy Thomas Bell, Esq., F.R.S. 
Received December 18, 1845, — Read March 12, 1846. 
The Porpoise inspires through the blow-hole which includes the nostrils, and is 
so placed that the animal can breathe with only a very small portion of its head 
above the water. A large passage leads down from the external opening to two 
canals that pass directly downwards through the skull, between the cranial and 
facial bones ; these nasal canals are separated from each other by a thin plate of 
bone (Plate XII. fig. 1.3-26.27)*. The two canals, after passing down through the 
skull, coalesce to form a muscular tube that opens at its lower part into the pharynx, 
by a constricted aperture. Through this aperture the mouth of the larynx rises, 
after the larynx itself has projected quite through the pharynx, dividing it into two 
channels. 
Connected with the channel that leads from the external opening down to the two 
bony conduits is a series of pouches ; these are composed of aponeurotic walls, 
folded on themselves in many wrinkles, so as to be capable of great distension 
The tv/o largest and most dilatable of these pouches are seated on each side and 
in front of the outer passage, into which they open separately (fig. 1.2-1). 
Deeper than, and between these two anterior lateral pouches, is the anterior central 
pouch, which is placed upon the combined maxillary bones, and communicates with 
the nasal canal by a large opening guarded by two oblong cartilages ; this sac has 
a muscular coat (fig. 1.2-2). 
The openings into the two nasal canals are just behind and below this anterior 
deep pouch. 
Immediately above the apertures of the two bony canals are two other horizontal 
apertures that communicate with two corrugated dilatable pouches (fig. 1.2-3). 
These posterior deep sacs are immediately behind the external channel, and are 
seated between it and the frontal bone ; their openings look downwards to the bony 
canals, their summits rise directly upwards in front of the frontal bones. 
The five pouches are under complete muscular control. 
When filled, their apertures can be closed, and their contents retained ; or the 
apertures can be thrown open, and their contents pressed out. 
The pouches, when distended with air, doubtless act so as to float the external 
* Figure 1 must be looked at, not in the direction of the plate, but sideways, 
t See Cyclopaedia of Anatomy, i. 580, fig. 269, Article Cetacea, by Professor Owen. 
