Vertebrata. 



345 



of the heart has proceeded further, the ventricle is at least imperfectly 

 divided, and in the Crooodilia is completely separated by a perfect 

 septum into right and left chambers, iu communication with the 

 corresponding auricles. The conus is rudimentary or absent. 

 Birds and Mammals exhibit conditions very similar to those iu 

 Crocodiles ; the auricles and ventricles are completely divided ; the 

 conus is absent. Valves which regulate the flow of the blood 

 are always present at the junction of the auricle and ventricle and 

 also in the couus, or when this is absent, at the limit of the ventricle. 

 Auricles, ventricles, and conus are composed chiefly of striated 

 muscle cells. 



It must be noticed that the aiu-icle and ventricle are simple during the 

 embryonic life of the higher Vertebrata, the septa being formed later. 



Fig. 285. Diagrammatic longitudinal section through the head and front end of the 

 body to show the position of the heart and the pericardium. A Fish, B higher 

 Vertebrate, c posterior boundary of the skull (the rest of which is not drawn), h ventricle, 

 I body-cavity, m buccal- cavity, o oesophagus, s septum or pericardium, v stomach. — Orig. 



The heart is ventral to the alimentary canal ; in Pisces it lies close to the 

 head in a special portion of the body-cavity (Fig. 285 A), closed ofE from the 

 rest by a transverse septum. In other Vertebrata it has moved further 

 back, and therefore this septum is sacculated, and forms a pouch round the heart, 

 the pericardium, which projects some way back into the general body-cavity. 

 A large ventral aorta springs from the heart in Pish and 

 gives off a branch to each gill-arch, the afferent branchial 

 arteries J there are usually five pairs altogether in Selachians 

 (gill-arches 2 — 6), and four pairs in the Teleostei (gill-arches 

 •3 — 6). The blood, after passing through the capillaries of the gills 

 of each arch, proceeds through the efferent branchial 

 arteries to the aorta, a large unpaired artery running below the 

 vertebral column, and giving oS branches to the various organs. The 

 large arteries of the head (carotids) arise from the first efferent 

 branchial artery ; if a functional lung is present, it usually receives 

 blood from a branch of the last efferent branchial artery. The 

 Amphibia generally exhibit similar arrangements during larval 

 life. Later the afferent and efferent arteries unite to form four 

 simple arterial arches on either side, which run directly into the 

 aorta. Of these, the third is usually atrophied in the adult ; the 



