THE GARDEN TOAD 13 



Examine the brain. In front of the true brain are the 

 olfactory lobes, the nervous centre for the sense of smell. 

 The brain itself is composed of several parts. The 

 anterior portion consists of two elongated parts, the 

 cerebral hemispheres; just back of these are the optic lobes 

 or midbrain, consisting of two short lobes, which are fol- 

 lowed by the small cerebellum, which in turn is followed 

 by a long part, the medulla oblongata, which runs imper- 

 ceptibly into the long dorsal nerve, the spinal cord. Note 

 the large optic nerves running out to each eye. How far 

 backward does the spinal cord extend ? Note the many 

 pairs of nerves given off from the brain and spinal cord. 

 These nerves branch and subdivide until they end in very 

 fine fibres. Some end in the muscle-fibres, and through 

 them the central nervous system innervates the muscles. 

 These are motor endings. Still others pass to the surface 

 and receive impressions from the outside. These last are 

 sensory endings. Note that the spinal nerves arise from 

 the spinal cord by two roots, an anterior or ventral, and 

 a posterior or dorsal root. Trace the principal spinal 

 nerves to the body-parts innervated by them. These 

 nerves are numbered as first, second, etc., according to 

 the number of the vertebrae (counting from the head back- 

 ward) from behind which they arise. 



