SUBPHYLUM VERTEBRATA 



327 



Amphibia; it appears in early stages in the development of still higher 

 forms and also becomes vestigial in the adults of these forms. The 

 metanephros is the functional kidney of the reptiles, birds, and mammals. 

 This, therefore, is another illustration of the biogenetic law. 



350. Nervous System. — The nervous system of a vertebrate consists 

 of a brain and a spinal cord, together forming the central nervous system; 

 and of nerves, ganglia, and sense organs, forming the 'peripheral nervous 

 system. The nerves which lead to and from the brain or spinal cord are 

 called respectively cranial or spinal nerves and form the cerebrospinal 

 system. They receive afferent impulses from various sense organs 

 which they carry to the appropriate center and efferent impulses which 

 they conduct from nerve centers to active organs, such as muscles and 



lV/7/fe 

 rnaffer. 



Spinal 

 cord 



Sensory 

 nerve 



Dorsal roof 

 Receptor neuron 

 ion 



Skin 



Recepfvr 

 (sense organ) 



Muscle 

 (effector) 



Ventral roof 



Motor nerve 

 Adjustor neuron fiber 



Fig. 218. — Diagrammatic cross section of spinal cord and diagram of a spinal nerve, 

 show reflex paths; should be compared with Fig. 142. 



To 



glands (Fig. 218). Peripheral ganglia connected with these and situated 

 in various parts of the body serve as local centers for the control of 

 certain localized activities. Ganglia of that character are similar in 

 function to the central ganglia of animals in the lower phyla. 



There is also a portion of the peripheral nervous system, including 

 both ganglia and nerves, which is to a considerable degree detached 

 from the rest and which carries on its functions mostly without any 

 interference from the central nervous system. For this reason it is 

 known as the autonomic nervous system. The nerve fibers belonging 

 to the cerebrospinal system run from origin to destination without 

 branching; this serves to keep the impulses separate and distinct, as is 

 necessary in all voluntary action. Those of the autonomic system, 

 however, branch freely, causing the effect of any stimulus to be radiated 

 in all directions. The system is thus very widely affected by stimuli, 

 which suggests the term sympathetic, also applied to it. This system 

 controls the involuntary muscles of the body. Its nerves run to and 

 from the various regions of the alimentary canal and through the cere- 

 brospinal nerves fibers from it reach all of the blood vessels throughout 



