Nervous Systems 825 



roots in the anterior medulla fish often show a continuous rapid rhythmic 

 movement of fins (Fig. 308).-"^ The pectoral fin rhythm is normally slower 

 than the dorsal; there is interaction between the two rhythms, but the 

 rhythm of one fin can be altered reflexly without affecting the other. A se- 

 ries of rhythmic centers in the medulla is postulated but the part played by 

 reflex re-excitation is not well known. 



In amphibians the spinal cord alone permits considerable locomotion. A 

 spinal frog jumps in coordinated fashion when stimulated but not spontane- 

 ously; reflex responses, particularly of protractor muscles, remain coordin- 

 ated. ^^^ If one or two legs are deafferented CBufo^ there is little interference 

 with coordinated ambulation; when three are deafferented coordination is 

 strikingly reduced. After deafferentation of all four limbs no locomotor move- 

 ments occur, although the toad can swim, provided the labyrinths are intact. 

 If only one leg retains its sensory and motor nerve supply and all other 

 sensory nerves are cut, there are normal diagonal limb movements; if the 

 motor root of, the fourth leg is then cut, all ambulatory movements cease, 

 and stimulation of this fourth leg elicits only simple monophasic responses 

 in the other three limbs. In the toad, therefore, sensory and motor supply of 

 one segment is necessary for the diagonal pattern of ambulation.^'*'' ^'^'^ 



In contrast to this strictly reflex picture of ambulation in Amphibia is the 

 concept of a fixed pattern in the central nervous system. The basic patterns 

 of coordination arise by self-differentiation within nerve centers prior to ex- 

 perience."*^* Embryos kept narcotized for several days emerge from their 

 narcosis at the behavior stage for their age without ever having experienced 

 the motions of the developmental stages corresponding to the period of nar- 

 cosis. In an extended research on salamanders Weiss*^'^- *-* has examined 

 the movement of limbs transplanted to abnormal body regions. When a su- 

 pernumerary fore-leg is transplanted to a position near a normal one and in- 

 nervated by one or a few nerves of the brachial plexus, the corresponding 

 muscles of the two legs always contract simultaneously. If a limb is rotated 

 by 180° when transplanted to replace a normal limb, the grafted Hmb moves 

 in perfect temporal coordination but in reverse direction to normal, with the 

 result that the transplanted forelimbs move the salamander backward. The 

 effects are similar when the limbs are deafferented. Each re-innervated mus- 

 cle of the extra limb contracts synchronously with the corresponding muscle 

 of the normal limb. The spinal cord signals a particular muscle irrespective of 

 its innervation, as if there were for a particular muscle a sort of "resonance" 

 pattern in the cord as a whole. Basic motor patterns owe their organization 

 to the intrinsic properties of the central nervous system rather than to peri- 

 pheral influences; furthermore, the patterns are permanent, stable, and non- 

 modifiable in their essential feaures. This contrasts with the mammals, in 

 which corrective influences (probably of cortical origin) can supersede the 

 original patterns of coordination if the latter have become inadequate. 



The two viewpoints, reflex control and intrinsic patterns, are not mutually 

 exclusive. The intrinsic pattern limits the range within which reflex control 

 can operate. The physiological basis for such limitation is unknown, and 

 there is a wide gap between our knowledge of simple spinal reflexes and 

 integrated locomotion. It is certain, however, that the spinal cord is much 

 more than a complicated switchboard and terms such as homologous func- 



