90 ESSENTIALS OF ZOOLOGY 



The pectoral muscles cover the chest and ventral portion of the 

 upper body region; the rectus abdominis extends along the median 

 ventral region; the paired ohliquus externus and internus cover most 

 of the sides of the trunk. The muscles of the limbs are numerous. 

 There are some eighteen separate muscles which control various move- 

 ments of the legs. A detailed description of these and other muscles 

 of the frog would be confusing to the elementary student and there- 

 fore is not included. 



Nervous System 



The three divisions in which the nervous system of the bullfrog 

 may be considered are: (1) central nervous system, (2) peripheral 

 nervous system, and (3) autonomic or sympathetic nervous system. 



The central nervous system, so called because it comprises the 

 larger number of nerve centers, consists of the brain and spinal cord. 

 The peripheral nervous system consists of (1) the paired cranial and 

 spinal nerves which connect the brain and spinal cord with other 

 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, and 

 organs of respiration. 



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

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

 sions, the forebradn, midbrain, and Jiindbrain. 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 lobes. 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 



