352 MANUAL OF ZOOLOGY sect. 



to the systemic arteries. A certain degree of mixing, how- 

 ever, of the venous and arterial currents takes place as they 

 pass through the ventricle. 



In the rabbit this mixing of the arterial and venous cur- 

 rents is entirely prevented, owing to the ventricle being 

 completely divided into two chambers — right and left. 

 The right auricle opens into the right ventricle, and fills it 

 with venous blood from the great veins. From the right 

 auricle the blood is driven through a pulmonary artery to the 

 lungs. From the lungs the oxygenated blood is returned 

 by means of the pulmonary veins to the left auricle ; from 

 the left auricle it enters the left ventricle, and from the 

 latter is driven out through the system of systemic arteries 

 to all parts of the body. There are thus two distinct cur- 

 rents of blood constantly passing simultaneously through the 

 heart, but entirely cut off from one another, viz., a venous 

 current on the right side and an arterial on the left. The 

 blood of the rabbit has a much higher temperature than 

 that of the dogfish or lizard. 



In all the three examples the veins which carry the 

 venous blood towards the heart from the stomach, intestine, 

 and pancreas unite together to form a large vein, the hepatic 

 portal, which ramifies in the substance of the liver, and 

 forms the main source of the blood supply of that organ. 

 In the dogfish and lizard, but not in the rabbit, veins con- 

 vey blood from the posterior region to the kidneys, forming 

 what is termed a renal portal system. 



The nervous system is highly developed. The central 

 nervous system consists of the brain and spinal cord. The 

 brain is, as already stated, contained in the cavity of the 

 cranium ; the spinal cord, continuous with the posterior end 

 of the brain, extends through the neural canal roofed over 

 by the series of neural arches of the vertebrae. 



