THE VERTEBRATE BODY 



321 



principles of muscle physiology that apply to all mammals have been 

 worked out with frog muscles. 



The Nervous System. This system has the general function of co- 

 ordinating the various parts of the body with one another so that the 

 body functions as a unit rather than as individual parts. For conven- 

 ience of study it is divided into the central nervous system, consisting of 

 the brain and spinal cord ; the peripheral nervous system, consisting of 

 cranial and spinal nerves ; the autonomic nervous system, a set of nerve 

 ganglia and nerves that control the involuntary body reactions ; and the 

 sense organs. 



As in all vertebrates, the brain of the frog is divided into five regions, 

 some of which are much more conspicuous than others. Part I consists 



Skin 



Sense ending 



Sensory fiber 



Assoaational neuron 



White matter 



Gray matter 



Central canal 



Dorsal root 



Ventral root 

 Motor fiber 

 Muscle 



Fig. 22.12. Cross-section through the spinal cord showing how connections may be 

 made in reflex action. The connections with nerve fibers going to and from the brain 



are not shown. 



of two elongated cerebral hemispheres with a pair of olfactory lobes at 

 their anterior ends — the olfactory nerves are attached to these. Part II 

 is the inconspicuous diencephalon, to which is attached the pineal body 

 dorsally and the pituitary gland ventrally. The optic nerves from the 

 eyes enter the ventral side of the diencephalon. These two regions are 

 often grouped together as the forebrain. Part III is the midbrain, 

 which bears the conspicuous optic lobes on its dorsal surface. Two 

 pairs of cranial nerves are attached to this region. (These are the 

 oculomotor and trochlear nerves which run to the muscles which move 

 the eyeballs.) Part IV is the cerebellum which is reduced to a thin band 

 of nervous tissue just posterior to the optic lobes. Part V is the elon- 

 gated medulla which is easily recognized by the large triangular opening 

 on the dorsal side. Parts IV and V are often grouped together as the 



