84- 



Vertebrata. 



branclies are covered by connective tissue, so that tlie ramifications 

 cannot be seen from without. On the inner surface of the lang there 

 is usually, as has already been mentioned, a delicate vascular net- 

 work; this is not, however, the case in most Pishj here the lung, 

 which is usually an unpaired organ, has no such network, and 

 therefore no respiratory function; it simply possesses the power of 

 diminishing the specific gravity of the animal, and is termed the 

 swim-bladder. A respiratory lung is only possessed by the 

 Dipnoi and a few others. The unpaired, usually tubular, portion 

 which connects the two lungs with the alimentary canal, the wind- 

 pipe (trachea), varies much in length, usually in correlation with 

 the form of the neck ; it is generally strengthened by cartilaginous 

 or bony rings, and in most Vertebrata (Amphibia, Reptilia, Aves, 

 Mammalia, and some Pisces) it opens ventrally into the alimentary 

 canal ; in most Pisces, however, it opens dorsally. In the anterior, 

 specially modified portion of the trachea, the larynx, there is, 

 in many forms (Anura, Lacertilia, Crocodilia, Mammalia), a pair 

 of projecting membranous folds, the vocal cords, which are set 

 in motion by the currents of air and thus produce the voice. 



Circulatory organs. In the Fish the heart consists of 

 three successive parts, atrium, ventricle, and con us 

 arteriosus.* The atrium is a thin-walled sac lying above the 

 ventricle which has thick spongy walls ; the conus is tubular. 



Each portion encloses a simple 

 cavity as does the large venous 

 sac, the sinus venosus, which 

 lies in front of the auricle and 

 brings into it the venous blood 

 from the body. In the Amphibia 

 the conus is well-developed, and the 

 ventricle, with its spongy walls, is 

 undivided, like that of Pisces. The 

 atrium, on the other hand, is 

 separated by a thin septum into 

 right and left auricles ; the sinus 

 venosus is also divided. The left 

 sides of the sinus and the corre- 

 sponding auricle receive the blood 

 from the lungs, the right that from 

 the rest of the body (for details see 

 Amphibia) . In the Reptilia 

 the atrium and the closely con- 

 nected sinus are divided just as in 

 the Amphibia; butthe specialisation 



Vig. 284. Heart of an Amphibian; 

 tie - ventral wall of atrium and yentricle 

 is removed by a horizontal aeotion ; 

 partly diagrammatic, h spongy wall of 

 the ventricle, fc valves, o opening of the 

 sinus venosus into the left auricle, o' 

 opening of the Sinus venosus into the 

 right auricle, s auricular septum, v wall 

 of auricle. — Orig. 



■ The conus is rudimentary in many Fish. 



