466 TEXT-BOOK OF EMBRYOLOGY. 



connected in various ways among themselves and probably having their own 

 reflex arcs or plexuses. These, ganglia are nevertheless to some extent under 

 the control of the efferent neurones of the central nervous system, some of 

 which send their axones to such ganglia (Fig. 404). There are thus in the 

 central nervous system two categories of efferent peripheral neurones, those 

 innervating visceral structures via sympathetic ganglia and those innervating 

 somatic structures. The bodies of the somatic efferent neurones are located 

 in the ventral gray matter of the nerve tube, while the bodies of the splanchnic 

 efferent neurones are believed to occupy more central and lateral positions in 

 the lower half of the gray matter of the neural tube (Fig. 404). It is uncer- 

 tain whether there are similar afferent splanchnic neurones in the sympathetic 

 ganglia, and thus distinct from those in the spinal ganglia, or whether these all 

 lie in the spinal ganglia and are consequently not fully differentiated from the 

 somatic afferent neurones. 



The muscular segmentation of the trunk has already been mentioned and 

 also the corresponding segmental arrangement of the spinal nerves. Local 

 extensions of this musculature and of its overlying cutaneous surface in the 

 form of fins and limbs cause corresponding increase in the size of those seg- 

 ments of the cord innervating them. This is due to the increased number of 

 afferent fibers and consequent increase in the dorsal white columns and in the 

 receptive dorsal gray columns, also to the increase in the number of efferent 

 peripheral neurones whose bodies occupy the ventral gray column (e.g., cervi- 

 cal and lumbar enlargements). (Compare also the differentiation in the 

 cervical cord and lower medulla of the columns and nuclei of Goll for the 

 lower extremities and those of Burdach for the upper extremities). 



In general, the intermediate neurones of the cord fall into two categories; 

 intersegmental (ground bundles), connecting cord segments, and those send- 

 ing long ascending bundles to suprasegmental structures (see pp. 472 and 473.) 



The Epichordal Segmental Brain and Nerves. 



The principal peripheral structures which exert a determining influence on 

 the structure of the epichordal brain are : The mouth, the respiratory apparatus 

 (gills and later lungs), and two specialized sensory somatic structures, the 

 acustico-lateral system and the optic apparatus. 



In the gills we have essentially a series of vertical clefts forming communica- 

 tions between the pharynx and the exterior, the intervals between the clefts 

 being the gill arches. The musculature of the gill arches is morphologicaUy 

 splanchnic (see p. 311). The gill or branchial musculature is in closer 

 relations with stimuli from the external world than is the visceral musculature 

 of the body. As a result of this the former is not of the smooth involuntary 



