CIRCULATORY AND RESPIRATORY SYSTEMS. 61 



It is commonly isolated, yet may communicate with oesoph- 

 agus, as in Lepidosteus (gar), or be obscurely cellular (pul- 

 monic), and at times employed in respiration, as in Lepido- 

 siren (mudfish). Hence the swim bladder appears in some 

 fishes to represent a rudimentary lung. 



Relations of branchial arches to stylo-hyoid ligament of man. 

 The elements in the hyoid bone of osseous fishes are as fol- 

 lows: basi-hyal, ^glosso-hyal, uro-hyal, epi-hyal, cerato-hyal, 

 stylo-hyal; brauchiostegal, basi-branchial, hypo-branchial, 

 thyro-branchial, cerato-branchial, epi-branchial, and pha- 

 ryngo-branchial bones. Operculum, or gill cover is com- 

 posed of opercular, pre-opercular, inter-opercular, and sub- 

 opercular bones. The mechanism of respiration: muscles 

 producing adduction and abduction of operculum. 



Batrachia. Heart composed of two auricles and a single 

 ventricle. The venous blood, collected by the ascending 

 and descending cavse, is thrown from the right auricle into 

 the single ventricle, thence through the pulmonary artery to 

 the lungs, to be returned by the pulmonary veins to the ven- 

 tricle to be sent through the aorta to the body. Thus the 

 blood of a batrachian is mixed in the pulmonic and greater 

 portion of the systemic circulation. The larval form of some 

 batrachians, as Eana (frog), breathe by gills, and in certain 

 forms, as Axolotl, and Proteus (salamander), these are retained 

 throughout life. The lung, when present, is a double, coarsely 

 cellulated sac. The mechanism of respiration: muscles of 

 deglutition. 



Eeptilia. Heart generally of two auricles and a single 

 ventricle. A rudimentary septum is seen partially dividing 

 the ventricle into two parts in Ophidia. The septum is per- 

 fect in the Crocodilia, making four cavities to the heart. 

 The general plan of the circulation as in Batrachia, excepting 

 Crocodilia. In these animals the venous blood contained in 

 the right ventricle is sent not only to the lungs through the 

 pulmonary artery, but also, through a second large trunk 

 which becomes the left or descending aorta, to supply the ab- 

 domen and lower extremities. That portion of the blood arte- 

 rialized and returned from the lungs is received in the left 



