The Frog: A Representative Vertebrate 69 



the body cavity by a sheet of peritoneum. This is the suhvcrtcbral 

 lymph sinus. The coeloni itself is really a large lymph space. A pair 

 of lymph hearts provide a pumping mechanism which aids in keeping 

 the lymph circulating. The anterior pair lie beneath the scapulae while 

 the posterior pair are located on either side of the urostyle. These 

 latter may be seen pulsating in the living animal. 



THE NERVOUS SYSTEM 



An animal as complex as the frog has a need for some mechanism 

 for the coordination of its many functions. One of the systems which 

 serves this need is the nervous system. This system may be divided 

 into three portions: the central, the peripheral, and the autonomic. 



The Central Nervous System. — The central nervous system is 

 the main controlling mechanism of the body. It consists of the brain 

 and spinal cord. The brain of the frog is relatively small and is con- 

 tained within the cranium. The anterior part is made up of the ol- 

 jactory lobes which are concerned with the sense of smell. From these 

 lobes, the olfactory tracts pass to the nose. Just posterior to these 

 lobes are the cerebral hemispheres. Together these are known as 

 the cerebrum. In the frog this structure is somewhat elongate and 

 relatively very small ; it is concerned with conscious activity. 



Posterior to the cerebrum is the small diencephalon or thalamus. 

 This structure contains nerve fibers which connect the anterior and 

 posterior portions of the brain. The dorsal part of the diencephalon 

 bears a small projection which extends dorsally upward between the 

 cerebral lobes. This is the epiphysis or pineal body. On the ventral 

 portion of the diencephalon may be seen the optic chiasma where the 

 optic nerves cross before entering the optic lobes. At the posterior 

 part of the ventral side of the diencephalon is another projection, the 

 injundibulum. At the end of the infundibulum is the hypophysis or 

 pituitary gland. This important gland secretes a hormone that con- 

 trols many of the bodily activities. Posterior to the diencephalon are 

 the two large dorsal optic lobes which are concerned with the sense of 

 sight. These appear to rest on the ventral midbrain which is largely 

 made up of nerve fibers for the passage of nerve impulses. 



Posterior to the midbrain and optic lobe area is the small trans- 

 verse cerebellum whose function is the coordination of muscular ac- 

 tivity. The posterior portion of the brain is the medulla oblongata 

 which passes through the foramen magnum of the skull and merges 



