CENTRAL OBGAN AND PEEIPHEHAL NERVES (PHYSIOLOGICAL). 35 



ing may give the impression of an extreme effect in proportion to the 

 irritation. If one turns a sea-urchin on its back, it begins at once to lay- 

 hold of the ground with the long suckers which coTer its whole body. 

 Each arm, however, contracts instantly, when it touches the ground. The 

 almost egg-shaped animal is thereby more closely drawn to the ground. 

 Then a peculiar thing happens, as Eomanes and Ewart have well described. 

 At one place, no matter which, apparently, the arms are more strongly con- 

 tracted. Immediately all the others lose their hold, and the animal turns 

 toward that side where the stronger contraction occurred. As a result of 

 this, other arms are brought into contact with the ground, and they contract 

 in turn, and the process is repeated until the sea-urchin stands upon edge, 

 and then new pedicels come into action and finally bring the animal into 

 its normal position. Here we have a purposive movement, apparently prac- 

 ticable only through minute care and reflection, which may be explained 

 by simple reflex processes: by the contraction in the muscles of the am- 

 bulaeral feet following excitation of their sensory nerves. That the move- 

 ment is a regulated one bespeaks a combination of the nerves of the am- 

 bulacra! feet. But in this simple experiment appears a new faculty, which 

 until now we have not mentioned among the properties of the central nerv- 

 oiTS apparatus: the cessation of motion when once the animal reaches a 

 position of rest. There must here be introduced an inhibition from the 

 center, otherwise one could not understand why the sea-urchin should not 

 keep on turning until tired out, since even in the normal position new arms 

 are always coming in contact with the ground. In fact, it is a property of 

 nerve-centers, everywhere recognized, that they are not only able to excite 

 movements, but also to prevent them. The mechanism is not yet clearly 

 understood. Doubtless such inhibitions are propagated, as are the move- 

 ments, from the ganglion first excited to the others. 



It would be very alluring to follow from these first ideas further along 

 the events in a given part of the nervous system, or to see what takes 

 place in the internal nervous arrangement in the production of a given 

 complicated action. However simple, though, may be the most primitive 

 nervous apparatus, regarded anatomically, such views are misleading. 



As the simplest central nervous arrangement, we can consider that 

 which is made up of centripetal sensory and centrifugal motor fibers, in 

 which it is agreed that the ends of the sensory nerve are in contact directly, 

 or through the mediation of a second cell, with the cell of origin of the 

 motor nerve. Such simple combinations are widely found in the inverte- 

 brates as well as in the vertebrates. They occur partly in the sympathetic 

 ganglia, partly also as direct reflex-paths in the central nervous system. 

 Absolutely isolated, simple reflex-centers are not yet known, but even the 

 smallest are in connection with others similar to them. Such a center is 



