526 TEXTBOOK OF ZOOLOGY 



organs of the body and (2) a large number of small nerve centers, 

 ganglia, distributed throughout the body. The sympathetic nervous 

 system is a part of the peripheral nervous system. It is made up of 

 a large number of small ganglia, two rows of which form the sym- 

 pathetic trunks on each side of the vertebral column and connect with 

 the spinal nerves. The branches of these sympathetic trunks connect 

 with numerous small ganglia throughout the tissues of the body. 

 This system controls and regulates primarily the involuntary move- 

 ments of such organs as the heart, digestive tract, glands, organs of 

 respiration, and walls of blood vessels. 



Central Nervous System.— The brain is covered with a pigmented 

 membrane known as the pia mater. The brain has three main divi- 

 sions, the forehrain, midbrain, and hindbrain. The forebrain consists 

 of a pair of elongated cerebral hemispheres, separated from each other 

 by a fissure, and two enlargements at the anterior end of the hemi- 

 spheres known as the olfactory lohes. These lobes are fused on the 

 dorsal side but separated by a groove on the ventral side. Immedi- 

 ately behind the forebrain is the diencephalon. On its dorsal sur- 

 face is a vestige of the pineal organ which was more developed in the 

 tadpole. On its ventral surface is the optic chiasma, a crossing of 

 the optic nerves formed by fibers from the right and left sides, each 

 crossing to supply the eye of the opposite side. Just behind the optic 

 chiasma is the inf undibulum, and somewhat behind this is the pituitary 

 body, or hypophysis. The pituitary is of dual origin, developing in 

 part from the diencephalon and in part from the roof of the mouth 

 cavity. The midbrain contains two large rounded optic lobes. The 

 ventral part of the brain below these lobes is the crura cerebri. 



The hindbrain consists of the cerebellum and the medulla oblon- 

 gata. The cerebellum in the frog is almost rudimentary and consists 

 of a transverse fold of tissue immediately posterior to the optic lobes. 

 The cerebellum is in close connection with the large triang-ular 

 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 embryological development the central cavity is large ; but, as 

 maturity is approached, the walls thicken, and the cavity, particu- 

 larly in the spinal cord, is much reduced. In the brain these cavi- 

 ties, known as ventricles, form a continuous channel for the flow of 

 cerebrospinal fluid. The ventricles are connected one with another 



