6 ANATOMY OF VERTEBRATES. 



the venous blood is mixed with the arterial blood which circulates 

 over the body. The lungs retain the form of bags, with cellulo- 

 vascular walls, varying as to thickness, and are situated, with the 

 other organs of vegetative life, in a common thoracic-abdominal 

 cavity. 



§ 5. Avian modification. — "When the lungs become spongy, and 

 the cavity of the air-bag is obliterated by the multiplication of 

 vascular cellules, and when a four-chambered heart transmits the 

 venous blood to the lungs, and pure arterial blood to the body, 

 the temperature is raised, and is maintained at from 90° to 105° 

 Fahr., whatever may be that of the surrounding medium. Of 

 these hot-blooded vertebrates, one class has the lungs fixed, and 

 communicating with air-cells extending into the abdomen, and 

 usually other parts of the body ; this class is oviparous, is clothed 

 with feathers, and has the pectoral limbs modified as wings ; it is 

 called Aves — Birds. 



§ 6. Mammalian modification. — In the other class of warm- 

 blooded animals, the spongy lungs are freely suspended and 

 confined to a thoracic cavity, defined by a midriff from the 

 abdomen ; the class is hair-clad, viviparous, and suckles the 

 young, whence it is called Mammalia — Mammals. 



§ 7. Genetic and thermal distinctions. — The broad and well- 

 marked characters afforded by the respiratory system will probably 

 give permanence to the division, so convenient for most purposes, 

 of the vertebrate province into the four great classes above 

 defined, viz. Pisces, Reptilia, Aves, Mammalia. 



But many important relations and affinities are thereby masked. 

 Although the last two classes agree, as ' hot-blooded vertebrates,' 

 in their higher cerebral developement, and in the more complex 

 heart and lungs, birds, by genetic and developmental characters, 

 as well as by the general plan of their organisation, are more 

 intimately and naturally allied to the oviparous saurians than to 

 the viviparous mammals. In their generation and development, 

 modern batrachians differ from other cold-blooded air-breathers, 

 and agree with fishes. Present knowledge of extinct forms more 

 clearly exposes the artificial nature of the primary groups of the 

 oviparous vertebrates. An important link, the Pterosauria, or 

 flying reptiles, with wings and air-sacs, fig. 108, more closely 

 connecting birds with the actual remnant of the reptilian class, has 

 passed away. Other extinct orders (Ganocephala and Labyrintho- 

 dontia) have demonstrated the artificial nature of the distinction 

 between fishes and reptiles, and the close transitions that connect 

 together all the cold-blooded vertebrates. 



