200 NERVOUS SYSTEM OF VERTEBRATES. 



nuclei and nerve components may be described under the general 

 name of visceral efferent structures. 



The visceral efferent nuclei in the spinal cord occupy a position 

 dorsal to the ventral horn, between it and the visceral afferent 

 column (Fig. io6). The visceral efferent nuclei in the higher 

 vertebrates and man form here a lateral projection of the gray 

 matter known as the lateral horn. The neurites from the cells 

 of this column pass out of the cord in the dorsal nerve roots in 

 lower vertebrates. In higher vertebrates a part of these fibers, 

 and in some cases all of them, pass out by way of the ventral 

 roots. The fibers then go through the white rami communi- 

 cantes into the ganglia of the sympathetic chain. Their further 

 relations will be given in the next chapter. 



In lower vertebrates the visceral efferent column of the cord is 

 continued forward as a column in the brain constituting the 

 nuclei of origin of the efferent roots of the X, IX, VII and V nerves. 

 In fishes, where the voluminous musculature of the gill apparatus 

 is to be innervated, this is a large and important column of gray 

 matter at either side of the ventricle as far forward as the cere- 

 bellar segment. In higher vertebrates, where the gills have been 

 lost, the column becomes less important and is divided into dorsal 

 and ventral portions. The former is the dorsal vago-glosso- 

 pharyngeal nucleus, and gives rise to efferent sympathetic fibers. 

 The ventral portion consists of discontinuous masses known as the 

 nucleus ambiguus, and gives rise to fibers which innervate striped 

 muscles (Fig. 107). The neurites from the cells of this column form 

 the visceral efferent component of the X, IX, VII and V nerves in 

 all vertebrates. In the gill breathing forms (Figs. 51, 63, 79) 

 these components run in the posttrematic ramus of each of the 

 branchial nerves. This ramus runs down along the anterior side 

 of each branchial arch and hence behind the gill slit. The fibers 

 in question are distributed to the muscles which control the gill 

 arches in respiratory movements. The component in the trigem- 

 inus supplies more specialized muscle in all vertebrates. In 

 fishes this component runs in the mandibular ramus of the trigem- 

 inus which holds the same relation to the mandibular arch and 

 mouth that the posttrematic rami of the branchial nerves hold 



