8o Elementary Zoology. 



frog breathes it fills the mouth with air; the mouth is then 

 closed and the external nares, while the muscles forming the 

 floor of the mouth force the contained air into the lungs. 

 Expiration is effected by the abdominal muscles, which press 

 upon the viscera, and so upon the lungs, expelling the air. It 

 appears, however, that the frog can also breathe by means of 

 the skin, which is, it will be remembered, supplied with blood 

 by the cutaneous artery, a branch of the pulmonary. Respira- 

 tion is essentially an exchange of gases between the blood and 

 the air in the lungs. The haemoglobin, which tinges the red 

 blood corpuscles, has the power of absorbing and entering into 

 loose combination with the oxygen drawn into the lungs ; this 

 oxygen is then given up with equal ease to the tissues through 

 which the blood passes after it has been through the lungs. 

 The carbonic acid which is there absorbed is given up to the 

 outside when the blood returns to the lungs. 



The nervous system consists of the central nervous system, 

 the brain and spinal cord, from which arise nerves which con- 

 stitute the peripheral nervous system. These nerves end 

 either in the muscles or in sense organs ; they are either motor 

 in function, or sensory. 



The central nervous system, unlike that of any of the 

 invertebrate types, is entirely dorsal in position. It runs in a 

 canal formed in front by the skull, and behind by the verte- 

 bral column. Furthermore, the central nervous system is 

 really a hollow tube, though the thickness of its walls far 

 exceeds the diameter of the lumen, save in certain regions of 

 the brain. By these two important facts the central nervous 

 system of all vertebrated animals can be distinguished from 

 the central nervous systems of other animals. 



The brain is divisible into several regions ; in front there is 

 an unpaired region from which arises the two olfactory nerves, 

 going to the nose ; this is the olfactory lobe. Behind this are 

 the paired cerebral hemispheres ; then follows a region, which is 

 depressed below the level of these, and is known as the 

 thalamencephalon. From the upper surface of the anterior 

 part of the thalamencephalon arises a short stalked body, the 

 pineal body ; this structure is the rudiment of an unpaired eye, 



