274 THE VITAL FUNCTIONS. 



sented on one side only, but in their relative situa- 

 tions 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 moUusca, 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 con- 

 vex margins of four cartilaginous or osseous arches, 

 which are themselves connected 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 

 surfaces are still more minutely examined, they 

 are found to be covered with innumerable minute 

 processes, crowded together like the pile of velvet ; 

 and on these are distributed myriads of blood- 

 vessels, spread, like a delicate network, over every 

 part of the surface. The whole extent of this sur- 

 face exposed to the action of the aerated water, 

 by these thickly set filaments, must be exceedingly 

 great.* 



A large flap, termed the Operculum, extends over 

 the whole organ, defending it from injury, and 

 leaving below a wide fissure for the escape of the 

 water, which has performed its office in respiration. 



* Dr. Monro computed that in the Skate, the surface of the gills 

 is, at the least, equal to the whole surface of the human body. 



