Nervous Systems 821 



arthropods represent fusion of several ganglia. The cephalopod brain-mass 

 represents fusion of paired cerebral, pedal, and visceral ganglia, and completely 

 surrounds the esophagus.-^^-- "*^^ Paul Bert in 1867 was the first to show that 

 much of the supraesophageal portion of the brain-mass was, like the associa- 

 tion areas in the mammalian cerebral cortex, electrically inexcitable with re- 

 spect to behavior. He also showed that a respiratory center exists in the 

 subesophageal portion of the brain-mass. This respiratory center has regions 

 for inspiration and expiration.^'^ A center in the subesophageal portion regu- 

 lates chromatophores. The subesophageal portion of the brain-mass'"^ con- 

 tains other local centers; the brachial and pedal ganglia controlling arms and 

 tentacles, the pedal ganglion controlling funnel and eye-muscles, and the pal- 

 liovisceral ganglion controlling mantle, fins, and viscera. In Octopus a cen- 

 ter for pupillarv closure is found in the subesophageal portion. ^-^ 



The supraesophageal portion-^^- contains: (1) Higher motor centers in the 

 circumesophageal region— lobus basalis anterior, posterior, and lateralis, lobus 

 pedunculi; stimulation of these lobes elicits movements of large groups of 

 muscles, and unilateral extirpation results in circus movements which may 

 be almost continuous. (2) Primary sensory centers such as the olfactory' 

 lobes and the optic lobes in the optic stalk; stimulation of the latter may 

 result in chromatophore expansion and some mantle and fin movement. (3) 

 The dorsally located verticalis complex of three lobes; electrical stimulation 

 of this results in no motor responses. After removal of the entire verticalis 

 complex Sepia can see, swim, steer properly, capture and eat prawns, etc., 

 but fails to follow prawns as they disappear out of sight. Removal of part of 

 the verticalis complex from Octopus hampered its ability to escape from a 

 cage and from a pan of shallow water. ^'^ The verticalis complex, then, is an 

 integrative portion of the cephalopod brain-mass. 



SPINAL REFLEX CONTROL OF VERTEBRATE LOCOMOTION 



In the more efficient, better coordinated invertebrates the peripheral por- 

 tions of the nervous system have lost autonomy, and behavior depends on 

 central nervous reflexes. In all classes of vertebrates locomotion is mediated 

 by spinal reflexes. In embryonic development, mass responses of large muscle 

 groups become differentiated into more precisely localized responses. In phy- 

 logenetic development the spinal cord is less autonomous as the control of 

 locomotion is pushed cephalad. Concurrent with cephalad development of 

 reflex centers there appears more antagonism between regions of the central 

 nervous system, one region inhibiting another, usually cephalad regions 

 inhibiting "lower" or caudal centers. 



The basic organization of the spinal cord changes with the functions it 

 must perform. ^^ In general, each segment of the cord receives two pairs of 

 roots; the dorsal roots contain afferent (sensory) fibers, and a few efferent 

 (motor) fibers, while the ventral roots contain eff^erent (motor) fibers, both 

 somatic and visceral (autonomic), whose cell bodies are in the ventral horn 

 of the cord. In Amphioxus there are very many uniform segments, dorsal and 

 ventral roots emerging alternately. In cyclostomes also the dorsal and ventral 

 roots alternate; they unite, forming mixed nerves in myxinoids but not in 

 petromyzonts. From lower to higher classes of vertebrates there is a reduc- 

 tion in number of spinal segments and in the size of the caudal portion of 



