346 



CHORD AT A. 



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. 



Pineal Stalk. 



4th Ventricle., 



Medulla 

 Oblongata. 



The spinal nerves are clearly seen lying in the dorsal wall 



of the abdominal cavity. The first spinal, called the hypo- 



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 vertebrae. 



