92 ESSENTIALS OF ZOOLOGY 



medulla oblongata which constitutes the most posterior part of the 

 brain and is continuous with the spinal cord. 



Internal Organization. — The central nervous system is hollow. In 

 the brain the cavities, known as ventricles, form a continuous chan- 

 nel for the flow of cerebrospinal fluid. The ventricles are connected 

 one with another 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 tri- 

 angular fourth ventricle in the medulla oblongata. Vascular nets of 

 blood vessels called chorioid plexuses are found in each of these four 

 regions, and most of the cerebrospinal fluid is derived 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 within 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 com- 

 prising 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 neur'ocoele, 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 or autonomic 

 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, oculomotor, troclilearis, trigem- 

 inus, dbducens, 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 in 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 



