460 - Multicellular Animals, Especially Man 



GRAY MATTER 

 OF CHORD 



WHITE MATTER 

 CENTRAL CANAL 



DORSAL ROOT OF SPINAL NERVE 



AFFERENT NERVE FIBER FROM 

 RECEPTOR IN SKIN OR BODY WALL 



■AFFERENT NERVE FIBER FROM 

 RECEPTOR IN VISCERA 



SENSORY GANGLION 



POSTGANGLIONIC FIBER 

 TO VISCERA. ARROWS 



INDICATE DIRECTIONS OF 

 THE NERVE IMPULSES 



EFFERENT NERVE FIBER 

 TO SKELETAL MUSCLE 



TRUNK OF 

 ixSPINAL NERVE 



PREGANGLIONIC FIBER 

 TO LATERAL GANGLION 



PREGANGLIONIC FIBER TO 

 COLLATERAL GANGLION 



POSTGANGLIONIC FIBER 



TO SMOOTH MUSCLE 



OR GLAND OF SKIN 



OR BODY WALL 



COLLATERAL 

 SYMPATHETIC GANGLION 



Fig. 25-13. Diagrammatic cross section of the 

 the sympathetic ganglia on one side of the body. 



25-12). The higher centers — particularly the 

 cerebral hemispheres — increase in size and 

 complexity from the lower to the higher ver- 

 tebrates, until in man they overgrow prac- 

 tically all other parts of the brain. In both 

 the cerebrum and cerebellum there is an 

 external layer of gray matter, called the 

 cortex, which is formed by an outgrowth 

 from the deeper gray matter and which com- 

 pletely covers the outer surface of these or- 

 gans. This cortex, in both the cerebrum and 

 the cerebellum, consists entirely of associa- 

 tion neurons, which relay impulses to and 

 from the underlying white matter. The cor- 

 tex neurons are extremely numerous; several 

 billion are present in the cerebral hemi- 

 spheres of man. Accordingly it is not possible 

 to trace the exact path of the impulses in the 

 higher centers; nor is it possible to predict 

 the precise outcome of a response if the reflex 

 arc involves a higher center of the brain. 



The cortical regions of the higher centers 

 are very important, because (1) consciousness 



spinal cord (man), showing connections with 



occurs only in relation to nerve impulses that 

 pass through the cerebral cortex; and (2) the 

 capacity of vertebrate animals to form con- 

 ditioned reflexes is localized mainly in the 

 cortical areas of the cerebrum and cere- 

 bellum. 



The Craniospinal Nerves: Somafic Reflexes. 

 All movements of the limbs and other ex- 

 ternal body parts are performed by skeletal 

 muscles, and the skeletal muscles are all 

 activated by motor neurons from the spinal 

 cord and brain. Thus the spinal nerves and 

 the cranial nerves are directly concerned with 

 all somatic reflexes, which determine the ac- 

 tivities of the skeletal muscles. 



The Spinal Nerves. The segmented character 

 of the vertebrate nervous system is clearly 

 shown by the arrangement of the spinal 

 nerves (Fig. 25-10). The spinal nerves origi- 

 nate from the spinal cord in bilaterally sym- 

 metrical pairs, and each spinal nerve in- 

 nervates all the receptors and effectors on one 

 side of one segment of the bod\. Each spinal 



