346 CHORDATA. 
completely mix and every organ would on contraction of the ventricle 
be supplied with mixed blood. On the other hand, greater efficiency 
would be attained if the arterial blood could be sent to the tissues 
generally and venous blood to the lungs, and this is practically the 
case. The ventricle contracts rapidly after the auricles, before the 
blood from the latter has had time to mix, and hence the first portion of 
the blood leaving the ventricle is nearly all venous, because the opening 
of the truncus inclines to the right. This passes up the wide passage 
to the pulmonary arches, and only when these are comparatively full 
does the next portion of mixed blood diverge up the smaller aperture 
to the top of the truncus arteriosus. Here it passes up the wide open- 
ings of the two systemic arches, whilst only the last and most arterial 
portion reaches the small aperture to the carotids, ensuring a supply of 
pure blood to the brain. 
Fig. 244.—DorsaL View OF BRAIN OF FROG. 
— Olfactory Nerve. 
Olfactory Lobe. 
Cerebral 
Hemisphere. 
Pineal Stalk. Optic Thalami. 
Optic Lobes. 
Cerebellum. 
4th Ventricle. Medulla 
Oblongata. 
The spinal nerves are clearly seen lying in the dorsal wall 
of the abdominal cavity. The first spinal, called the Aypo- 
glossal, lies ventrally to the tongue, and can be 
seen on removal of the mylohyoid muscle. It 
joins the spinal cord between the first two vertebre. 
Nervous. 
