204 NERVOUS SYSTEM OF VERTEBRATES. 



arches has been retained as a girdle for the attachment of the 

 fore limb. The muscles which moved this branchial arch, or 

 perhaps those of several arches, have in part been preserved as 

 muscles of the limb girdle. These muscles and their nerves were 

 of course visceral muscles and nerves like those of other branchial 

 arches, and they have secondarily acquired somatic functions. 

 This interpretation is supported by the following facts: (i) The 

 position of the brachial plexus in mammals shows that the shoulder 

 girdle has shifted backward from its primitive position. (2) The 

 most primitive vertebrates now possess a large number of gills, 

 as many as thirty-five (Price). (3) There are in amphibian 

 embryos signs of gill slits extending back into the trunk region 

 caudal to the position of the fore limb (Platt). 



As pointed out in the last chapter, the somatic and visceral 

 efferent nuclei differ in the source of the impulses which come 

 to them and in the tracts which bring them. Tracts from higher 

 brain centers bring impulses to both sets of motor nuclei, but 

 much remains to be done in order to explain the mechanisms by 

 which somatic and visceral activities are correlated. 



Collaterals from afferent visceral fibers directly to the visceral 

 efferent nuclei are probably present in mammals (see Figs. 52 

 and 78). The short viscero-motor connections described in a previ- 

 ous chapter (p. 162) form a two linked chain between the visceral 

 sensory and the visceral motor apparatus. The tertiary connec- 

 tions of the inferior secondary gustatory center are not known. 

 From the superior secondary gustatory center a large tract goes 

 to the inferior lobes of the diencephalon, from which tracts go 

 to the cerebellum and medulla oblongata. The greater part 

 of the tract to the medulla oblongata ends in the region of the 

 visceral motor nuclei and it is undoubtedly chiefly these nuclei 

 which receive the impulses. Olfactory impulses also may come 

 over the same tract to the visceral motor nuclei. It is probable 

 that even in fishes tracts from other correlating centers, such as 

 the cerebellum or the mesencephalic nuclei, bring impulses to the 

 visceral motor nuclei for the control of some of the more complex 

 movements, especially for the coordination of somatic and visceral 

 muscles in the act of seizing food. 



