138 



Basic Structure of Vertebrates 



epidermis 



taste-bud 



Fig. 145. Diagram showing the central origin (from medulla) and the periph- 

 eral distribution of systems of nerve components. (From Goodrich: "Studies 

 on the Structure and Development of Vertebrates." By permission of The Mac- 

 millan Company, publishers.) 



reflect the general plan of the body — an outer somatic tube and an 

 inner visceral tube. 



A distinction between "general" and "special" nerves doubles 

 the number of classes of nerves. "General" is applied to nerves which 

 are related to numerous widely distributed structures of the same gen- 

 eral sort; e.g., the myomeres, the various sensory receptors distributed 

 throughout the skin. "Special" nerves innervate organs which are 

 more or less definitely localized and highly specialized; e.g., the eye, 

 ear, and olfactory organ, the extrinsic muscles of the eyeball. In each 

 of the four main classes of nerves defined above, both "general" and 

 "special" nerves may be recognized. 



The central nervous system consists of the brain and spinal cord. 

 The remotest outposts of the nervous system are the sensory receptors, 

 wherever situated, and the nerve-terminations on all effectors. Be- 

 tween the central organs and the peripheral termini of the system is an 

 overwhelmingly complex system of nerves whose ramifications extend 

 into every part of the body. For the most part, these nerves consist of 

 fibers which pass from the periphery directly and without interruption 



