On the Structure and Functions of the Organs of Respiration. 225 



system, by the action of the heart, through the respiratory apparatus, to 

 be submitted to the action of the air that is contained so scantily in the 

 water. The former of these ends is efiected by the connection of the gills 

 with the cavity of the mouth, the muscles of which send a rapid current of 

 water through the branchial passages ; and the latter, by the position of 

 the heart, which is placed so as to affect the respiratory organs previously 

 to the system at large. The gills in most fishes are disposed in fringed 

 laminae, the fibres of which are set close to each other, like the barbs of a 

 feather, and which are attached on each side of the throat, in double rows, 

 to the convex margins of four or five osseous or cartilaginous arches which 

 are very similar to the ribs. The extent of surface exposed by these gills 

 is very great. Dr. Monro computed that in the skate the surface of the 

 gills is at least equal to that of the human body. The water impelled 

 through them finds its way out by one large opening on each side of the 

 neck in the osseous fishes, or by five small ones in the cartilaginous species. 

 These are called branchial openings ; and it has been asserted by several 

 continental anatomists, that analogous openings may be very distinctly 

 seen, at an early period of the development of all higher animals, not 

 excepting man himself ; thus adding a very beautiful confirmation, if it 

 were necessary, to the general law that each of the systems of higher 

 animals, in the progress of its development, passes through a series of 

 forms analogous to those which remain permanent in the lower parts of 

 the scale. 



In considering the respiratory organs of fishes, we must not omit the 

 air bladder, which is now generally regarded as the rudimentary state of 

 the complex lungs of the higher verlebrata. In many fishes, as in the em- 

 bryo of mammalia, it is a simple shut sac, placed along the middle of the 

 back. In others it has a division of its cavity by one or more membran- 

 ous partitions. This air bag usually communicates with some part of the 

 alimentary canal near the stomach, by means of a short wide canal termed 

 the ductus pneumatic us ; but sometimes, as in the sword fish, it has no ma» 

 nifest opening ; and we find connected with it a glandular and highly vascular 

 organ, which has been supposed to secrete the gas which it contains. The 

 uses of the air bag are involved in great obscurity. It cannot be necessary 

 to regulate the specific gravity of the fish, for we often find it absent. The 

 gas contained in it differs from atmospheric air in its constitution : in the 

 fishes which inhabit deep seas, which are generally more muscular and 

 powerful than those which live near the shore or in fresh water^ there is a 

 considerable preponderance of oxygen ; and the proportion of nitrogen is 

 increased in those which come often to the surface. The diodon, or globe 

 fish, has a very large sac communicating with the oesophagus, which it has 

 the power of filling by swallowing air at the surface of the water, by which 

 means the whole body is blown up, and rendered tense and spherical, the 

 Bpioes covering it are erected, and the animal floats on the sea, quite secure 



