HOW FISHES BREATHE. 
345 
purposes which are served by their jointed arrangement. 
1 . Each half-arch, in being set on at an angle with the median 
chain of tongue-bearing bones (see Fig. 2), can, by the action 
of special sets of levator and depressor muscles,* * * § be made to 
form a greater angle with this middle base line, and so simul- 
taneously bring about ■ both the widening of each entire arch 
in its transverse diameter, and the increase of the space 
between it and its neighbours in front and behind. 2. It is 
possible for the upper and lower pharyngeal teeth-plates (_p 6, 
ip, Figs. 2 and 3) to act upon each other, and so be the means 
of a mastication of food in some sort.} 
Let us now briefly turn our attention to the spiny ruff 
which surrounds the throat of the fish. The bulk of this is 
formed on either side by two bones, the epi - and cerato-hyals , 
jointed on, one behind the other {eh, c h, Fig. 4), the former 
and most posterior of which is brought into relation with a 
bone — the “ hyo-mandibular ” — which is the chief support of 
the lower jaw, through the medium of a small bone, the 
66 stylohyal while the most anterior, the “ cerato-hyal,” is 
hinged on to the chain of tongue-bearing bones, not far behind 
its tip (* Fig. 2). This arch, like those which carry the gills, 
has, therefore, a very fair range of motion. The epi- and 
cerato-hyals further give support to certain curved bones (b s, 
Figs. 1 and 4), which afford attachment to an intervening 
membrane, in the manner of the ribs of an umbrella. These 
vary in number} and in mode of attachment, being hung on, 
sometimes to the outer, sometimes to the inner sides of their 
supports — why, it is not easy to say — and are called, from 
their office, 66 branchiostegal rays.”§ From ray to ray of this 
apparatus passes a muscle, which is attached behind to the 
inner surface of the operculum. This, by raising the rays, and 
acting at the same time in antagonism to a depressor muscle 
which rises from the cerato-hyal of the opposite side, and is 
inserted into the foremost of the rays, spreads out the bran- 
chiostegal membrane umbrella-fashion. “ These muscles,” as 
* The levators, depressors, and retractors of the hranchia. See Cuvier, 
Hist. Nat. des Poissons, tome i. pi. v. and vi. 
t “The necessary co-operation of the jaws,” observes Prof. Owen, “with 
the hyoid arch in the rhythmical movements of respiration is incompatible 
with protracted maxillary mastication ; and, accordingly, the branchial appa- 
ratus renders a compensatory return by giving up, as it were, the last pair 
of its arches to the completion of the work which the proper or anterior 
jaws were compelled by their services to respiration to leave unfinished.” 
X In the Syngnathus (Figs. 6 and 7) they are entirely wanting. Polyp- 
terus has but one, and there are only three present in Cyprinus. 
§ From the Greek Bpay^ia, gills, and orsyeiv, to cover. 
