78 
plus and minus in simplicity and complexity of vas- 
cular organization. ‘The heart in fish is found to 
differ in position and in arrangement of valves, 
which are more numerous in the cartilaginous than 
in the osseous fishes. The heart in amphibia is 
larger than in fish, smaller than in birds and quad- 
rupeds. The batrachia approach nearest to the fishes. 
The circulation is more complex in tortoises, and in 
lizards, and in serpents. In birds the warmth of 
blood gives clear indication of extended develope- 
ment of the respiratory and vascular system. Here 
are distinct pulmonary and aortal hearts; and not a 
part merely, but the whole of the blood is exposed 
to the action of air. Yet certain differences enable 
the anatomist to trace a connection with the pre- 
ceding class, the transition in the form of the heart, 
and the distribution of the vessels, particularly from 
the lizards. Respecting the veins of birds, their 
parietes are thicker than in other animals; and the 
enlargement of the inferior caval trunks, which Cu- 
vier and Meckel have observed in diving birds, ex- 
plains the long continued interruption of respiration 
which these birds can sustain*. Amongst mamma- 
lia, some have the circulatory system solely adapted 
to continuous access of air to the lungs; some, like 
diving birds and tortoises, have an organization 
adapted to long continuance under water. This is 
particularly noticeable in the porpoise and the seal. 
The foramen ovale in. the septum of the auricles, 
* Carus, p. 294. 
