ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM 109 



The centers or nuclei within the brain are of two general sorts : 

 (1) primary centers and (2) correlation centers. The primary 

 centers are directly connected with peripheral nerves, either as 

 terminal nuclei of afferent fibers or as nuclei of origin of efferent 

 fibers (see pp. 42, 108). The elements out of which most acts are 

 compounded are reflexes (see p. 56), and in the simplest of these 

 reflexes a sensory nervous impulse received from the periphery 

 by a terminal nucleus may be passed on to a nucleus of origin and 

 thence directly to the organ of response; but in more complex 

 reflexes the incoming nervous impulse is first transmitted from 

 the terminal nucleus to a correlation center, where it may meet 

 other types of sensory impulses and then be discharged into any 

 one of several possible motor pathways. For illustrations of 

 these types of connection see Chapter IV. 



In general, ganglia or nerve-centers are interpolated in con- 

 duction pathways only where some complication of the reaction 

 is to be provided. The conduction path is usually here inter- 

 rupted by synapses and various forms of correlation or coordina- 

 tion mechanisms are present (see p. 35 and Chapter IV). 

 Many of the sympathetic ganglia provide the mechanism for 

 local reflexes in which the central nervous system does not par- 

 ticipate (p. 225). The spinal ganglia (see Fig. 1, p. 25) are 

 often regarded as merely trophic centers for the maintenance of 

 the fibers of the peripheral nerves; but they evidently have 

 functions of correlation in addition to this, for numerous syn- 

 apses between sympathetic and cerebro-spinal neurons occur 

 here (see p. 228 and Fig. 109) which play a part in the correla- 

 tion of visceral and somatic reactions. 



The primary centers and the simpler correlation centers of the 

 brain can be studied much more readily in the brains of fishes, 

 which lack the cerebral cortex whose enormous development in 

 the human brain has obscured the relations and connections of 

 the more primitive reflex apparatus. Figures 42, 43, and 44 illus- 

 trate the relations of the principal sense organs to the brain in a 

 small shark, the common marine dogfish. Figures 42 and 43 

 (on the right side) illustrate the arrangement of the principal 

 roots and branches of the cranial nerves. On the left side of 

 Fig. 43 the relations of the nose, the eye, and the ear to the 



