MAMMALIA. 311 
ventricle to the auricle. There isin the adult no direct com- 
munication between the two sides of the heart. The course of 
the circulation is indicated in the subjoined diagram, and is 
shortly as follows: The venous blood, which has become im- 
pure by passing through the tissues, is returned by the great 
veins to the v7gh¢ auricle, from which it passes into the right 
ventricle. From here it is driven through a great vessel, called 
the pulmonary artery, to the 
lungs, where it is submitted 
to the action of the air, and 
becomes arterial blood. It is 
then returned to the heart by 
a series of vessels called the 
pulmonary veins, and is poured 
into the /ef¢ auricle, from 
which it passes into the left 
ventricle. From the left ven- 
tricle it is propelled to all parts 
of the body by a great systemic 
vessel, which is called the aorta 
(fig. 169). 
The lungs of Mammals are 
two in number, and differ 
from those of birds in being 
freely suspended in membran- 
ous bags. They are spongy 
and cellular throughout, and 
they never communicate by Fig, 169.-—Diagram of the circulation 
apertures on their surface ina Mammal. (The cavities contain- 
ith : l ad am dif ing venous blood are marked black, 
wa air-sacs place i ~ those containing arterial blood are 
ferent parts of the body. left white.) a@ Right auricle; v 
The xervous system of Mam- Right ventricle; Pulmonary 
artery carrying venous blood to the 
mals is chiefly remarkable for {anes day Palannsvy belie! catty: 
the great proportionate de- ing arterial blood from the lungs ; 
i ‘Left auricle ; w’ Left ventricle ; 
In, as x ; ; 
Raley mene Be then) bra sy 6 Aortacarrying arterial blood to the 
compared with the spinal body ; c Vena cava, carrying venous 
cord. blood to the heart. 
In the higher Mammals, also the two halves (hemispheres) of 
the brain proper (cerebrum) are connected together by a great 
band or bridge of nervous tissue, constituting what is known 
as the corpus callosum. This structure is not a conspicuous 
