12 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER II. 

 GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 



In any vertebrate animal two chief parts of the nervous system 

 are distinguished, the central system consisting of the brain and 

 spinal cord and the peripheral system. The peripheral system 

 includes the nerves which connect the central system with various 

 parts of the body, the ganglia of those nerves, sense organs, and 

 the sympathetic system. The central nervous system is situated 

 dorsal to the alimentary canal and is surrounded by more or less 

 strong skeletal structures which constitute the greater part of the 

 skull and spinal column. 



In the trunk region the central nervous system consists of a 

 rounded cord which is enclosed within the neural arches of the 

 vertebrae. In fishes this spinal cord extends the whole length of 

 the spinal column. In higher forms the caudal portion is less 

 developed, the cord as a whole grows less rapidly than the trunk 

 and remains in the adult shorter than the spinal column, ending 

 in the lumbar region. Beyond this a slender thread continuing 

 into the tail represents the caudal portion of the spinal cord of 

 fishes. In all vertebrates having well developed limbs the two 

 regions of the spinal cord with which the nerves of the limbs are 

 connected are somewhat thicker than the rest of the cord. These 

 thickened portions are known as the thoracic and lumbar enlarge- 

 ments. The cord is usually more or less flattened dorso-ventrally 

 and upon the dorsal and ventral surfaces in the middle line are to 

 be seen longitudinal grooves, the dorsal and ventral fissures. The 

 ventral fissure is usually a deep furrow r . If the cord be cut across 

 and the cut surface' examined with a hand lens, the greater depth 

 of the ventral fissure will be evident and in the median plane there 

 will be seen a narrow opening. This is the central canal wiiich 

 extends throughout the length of the cord. The material surround- 

 ing the central canal and filling up the inner portion of the cord 



