244 THE SCIENCE OF LIFE. 



efferent nerve conducts power for the muscles to contract. 

 If the afferent nerve of a part is cut across or injured, 

 sensation is lost, but motion remains ; but if the efferent 

 nerve is cut, the power of motion is lost while sensibility 

 continues. This form of nerve-action is called reflex. 

 Many actions of this sort are wholly involuntary, as the 

 motions of the limbs in paralytics excited by tickling the 

 soles of the feet. 



In the Star-fish we find a nervous ring around the 

 mouth, made of five ganglia, with radiating nerves, cor- 

 responding with the type of structure. The Mollusks 

 have an irregularly scattered nervous system, consisting 

 of two or more ganglia around the gullet and one or two 

 more in the posterior region, united by threads, and 

 sending fibers to various organs. The Articulates have 

 generally a double nervous cord along the ventral side, 

 studded with ganglia of nearly uniform size, except the 

 first, which is largest. In the higher forms, as the Bee, 

 several ganglia are fused together in the head and tho- 

 rax, indicating a concentration of organs for sensation 

 and locomotion. 



The nervous system of the Invertebrates is Jiomolog- 

 ically represented by the ganglionic or sympathetic sys- 

 tem of Vertebrates, which supplies the unstriped or in- 

 voluntary muscles, and presides over organic or visceral 

 functions, such as digestion and circulation. In addition 

 to the sympathetic system, Vertebrates have a brain and 

 spinal cord, forming the cerebro-spinal system, (Fig. 142,) 

 to which there is nothing similar in other animals, and 

 which presides over what are called the functions of ani- 

 mal or sentient life, as sensation and locomotion. Yet 



