GENERAL SOMATIC SYSTEMS OF CONDUCTION PATHS 181 



lar formation (p. 158) for reflex connections with the motor 

 nuclei at various levels. The arrangement of these motor nuclei 

 of the brain stem, from which peripheral motor fibers of the 

 cranial nerves arise, is shown on the left side of Fig. 71 (p. 154). 

 The details of these connections for local motor reflexes will not 

 be entered into here. From the ventral part of the thalamus 

 (p. 163) there are descending thalamo-bulbar and thalamo- 

 spinal tracts for local thalamic reflexes. The main descending 

 pathway for voluntary motor responses to general somatic 

 stimuli arises from the precentral gyrus of the cerebral cortex 

 (p. 283). This is the tractus cortico-bulbaris (Fig. 75) and trac- 

 tus cortico-spinalis or pyramidal tract (Figs. 64, 75, 137). The 

 reflex connections effected in the medulla oblongata are some- 

 what more complex than those of the spinal cord, that is, they 

 represent the integration of more different kinds of sensory im- 

 pulses and facilitate the performance of a greater variety of 

 movements by way of response. Similarly, the complexity of 

 the reflex adjustments increases as we pass forward into the mid- 

 brain, thalamus, and cerebral cortex (see p. 63). 



Attention has already been called to the fact that the centers of adjust- 

 ment in the brain stem are of two physiologically different types which we 

 have termed centers of correlation and centers of coordination (p. 35). The 

 more labile and individually variable adjustments are effected in the corre- 

 lation centers which are developed from the more dorsal parts of the embry- 

 onic neural tube above the limiting sulcus (p. 120), while the more ventral 

 parts of the neural tube give rise to the motor centers and the centers of co- 

 ordination, whose adjustments are of a more fixed and invariable character. 

 In the embryonic development the coordination centers develop preco- 

 ciously, while the correlation centers mature more slowly; the higher asso- 

 ciation centers of the thalamus and cerebral cortex in particular are the last 

 to mature (p. 286). 



In the phylogenetic development of the brain the same rule holds. In 

 the lowest vertebrates the coordination centers are much larger in pro- 

 portion to the size of the correlation centers than in higher vertebrates. 

 Bartlemez 1 has analyzed these motor coordination mechanisms (which he 

 terms in the aggregate the nucleus motorius tegmenti) in fishes, and finds in 

 the motor tegmentum throughout the medulla oblongata a nucleus of a 

 primitive type whose neurons serve to connect the primary sensory nuclei 

 with the primary motor nuclei. Some of these connections are very short, 

 while others are very long, reaching remote parts of the brain and spinal 

 cord through the longitudinal medial fasciculus (pp. 185, 211). This nu- 

 cleus is the parent tissue out of which the more complex coordination centers 

 in the tegmentum of higher vertebrates have been differentiated. 



1 BARTLEMEZ, G. W. 1915. Mauthner's Cell and the Nucleus Motorius 

 Tegmenti, Jour. Comp. Neur., vol. xxv, pp. 87-128. 



