216 



THE VITAL FUNCTIONS. 



tion assumes a higher importance. In fishes the gills form 

 large organs, and the continuance of their action is more es- 

 sential to life than it appears to be in any of the inferior 

 classes: they are situated, as is well known, on each side of 

 the throat in the immediate vicinity of the heart. Their 

 usual form is shown at G G, Fig. 3S9^ where they are repre- 



sented on one side only, but in their relative situations with 

 respect to the auricle (D,) and ventricle (E,) of the heart; the 

 bulbus arteriosus (b,) and the branchial artery (F.) They 

 have the same fringed structure as in the mollusca, the fibres 

 being set close to each other, like the barbs of a feather, or 

 the teeth of a fine comb, and being attached on each side of 

 the throat, in double rows, to the convex margins of four 

 cartilaginous or osseous arches, which are themselves con- 

 nected with the jaws by the bone called the os hyoides. The 

 mode of their articulation is such as to allow each arch to 

 have a small motion forwards, by which they are separated 

 from one another; and by moving backwards they are again 

 brought together, or collapsed. Each filament contains a 

 slender plate of cartilage, giving it mechanical support, and 

 enabling it to preserve its shape while moved by the streams 

 of water which are perpetually rushing past. When their 



