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POPULAR SCIENCE REVIEW. 
Professor Owen observes, 44 regulate the capacity of the branchial 
chamber, and mainly act upon the water it contains.” 
The only point to be noted about the operculum is, that it 
consists of four elements (see Fig. 4) fastened together, so as 
functionally to be one scale-like bone. This can be set ajar, 
door-wise, or be brought close to the head by the action of 
special muscles ; the hinge, so to speak, being at its anterior edge. 
Having now described the apparatus for branchial breathing, 
let us take an anatomical glance at the method by which a 
single respiratory act is effected in an osseous fish. 
This act 44 differs,” as Professor Owen well observes, 44 from 
that of swallowing, only in the streams of water being pre- 
vented from entering the gullet, and being diverted to the 
branchial slits on each side the pharynx.” First, the mouth is 
opened by the drawing back of the maxillary bones, and the 
pulling downwards of the mandible by their proper muscles, 
while at the same time the two 44 rami,” or branches, of the 
latter bone are separated wider behind by the action of certain 
muscles on its supports (the 44 hyo-mandibular ” arch). The 
result of this latter act is the widening of the branchial cavity, 
the space of which is further increased by the opening of the 
opercular doors, and by the spreading out of the brancbio- 
stegal rays, and consequent stretching of the membrane ex- 
tended between them. The branchial arches are, moreover, 
drawn forward by the action of their levator muscles, i.e. are 
forced to make a greater angle with the median chain of bones 
than they do when at rest. During this time the water, whose 
reflux from the mouth is barred by the action of the 44 velum ” 
already described, rushes through the teeth-guarded sluices* of 
the branchial arches, and washes over the gill-plates, freely 
distributing the stores of life with which it is laden. Next, 
the fissures leading to the gill-chamber are closed by the 
depression, through the action of special muscles, of the bran- 
chial arches, and by the pushing forward of the cerato-hyals by 
muscles which pass from these bones to the inner side of either 
branch of the mandible, not far from its tip.f The water, 
then — being pent up in the gill-chamber, and all return to the 
* Were it not for these teeth the fish would he speedily choked by 
various substances going “the wrong way” into the gill-chamber. The 
Mullet, which takes in a quantity of sand and mud with its food, and works 
it about between its pharyngeal bones, has a number of extremely delicate 
hinges attached to the concavity of its branchial arches. 
t The reciprocal action of these muscles — the homologues, according to 
Cuvier, of the geniohyoidei in ourselves — is extremely interesting. When 
the cerato-hyals are the fixed points from which they act, they depress the 
mandible; when, however, this latter bone becomes the fulcrum, they pull 
forward the hyoid apparatus, through the medium of the cerato-hyals. 
