CLASS AMPHIBIA 527 



by openings known as foramina. The cavities are large in four 

 regions: (1) the paired lateral ventricles in the cerebral hemispheres, 

 (2) the single third ventricle in the diencephalon, (3) the paired 

 optic ventricles in the optic lobes, (4) the single large triangular 

 fourth ventricle in the medulla oblongata. Vascular nets of blood 

 vessels in the much-folded pia mater constitute clioroid plexuses 

 that form the roofs of the third and fourth ventricles and extend into 

 the other ventricles somewhat. Most of the cerebrospinal fluid is de- 

 rived from the blood vessels of these plexuses. 



The spinal cord is continuous with the medulla oblongata ante- 

 riorly, runs posteriorly through the canal formed by the vertebrae, 

 and finally tapers to a narrow filament which ends in the urostyle. It 

 is covered by two membranes, an outer dura mater and an inner pia 

 mater. It is somewhat flattened, and a median fissure occurs on both 

 its dorsal and ventral sides. The central part of the cord comprising 

 its bulk is made up of gray matter consisting primarily of nerve 

 cells. In the center of this gray matter is a small hollow canal, the 

 neurocoele, which communicates with the ventricles of the brain. 

 Surrounding the gray matter is white matter consisting chiefly of 

 nerve fibers. 



Peripheral Nervous System. — The peripheral nervous system is 

 composed of the cranial, spinal, and sympathetic nerves, the last of 

 which will be considered separately. 



The cranial nerves arise from the brain, and there are ten pairs of 

 them in the bullfrog. Counting from the olfactory lobes backward, 

 they are as follows: olfactory, optic, ocidomotor, trochlearis, trigem- 

 inus, ahducens, facial, auditory, glossopharyngeal, and the vagus. 

 All of these, with the exception of the tenth or vagus nerve, run to 

 parts of the head. The vagus nerves branch to the heart, lungs, and 

 digestive system. 



The bullfrog has ten pairs of spinal nerves. Each spinal nerve 

 originates from the gray matter in the spinal cord by a dorsal and 

 a ventral root. These roots pass out of the vertebral column between 

 vertebrae through an opening or intervertebral foramen and unite 

 into a nerve trunk, branches of which extend to the muscles and skin 

 of the body and limbs. The dorsal root is known as the sensory or 

 afferent root and has a ganglion; the ventral root is known as the 

 efferent or motor root and has no ganglion. "Where these roots meet 



