THE NERVOUS SYSTEM. 513 



The visual apparatus, eye, with the eye-moving III and IV mid-brain nerves 

 as its efferent portion. (3) The general sensory apparatus of the surfaces of 

 the head and mouth, the afferent trigeminus, with the jaw-moving efferent 

 trigeminus. (4) The auditory (and vestibular) apparatus, the ear (VIII 

 nerve), with the VI (turning the eye to the source of sound) and VII (ear and 

 face muscles) efferent nerves. In the latter, the original ear-moving apparatus 

 has been replaced largely, in man, by the muscles of expression. " (5) The 

 visceral segment (IX, X, and XII nerves) , not indicated externally in forms with- 

 out gills. The afferent portion is concerned with taste and visceral stimuli, 

 the efferent with tasting, swallowing, sound-production and other visceral 

 functions. Overlapping with other segments is due to its visceral as opposed 

 their somatic chracter. The apparent dislocation shown by the abducens is 

 due to its common use by more than one segment. 



Caudal to this is the mechanism for head movement (N. XI), its afferent por- 

 tion being the upper spinal nerves. Following this, there is the segmental 

 series of spinal nerves which in places shows a tendency to fuse (plexuses) into 

 larger segments (phrenic segment, limb segments). All such modifications are 

 expressions of more recent functional adjustments modifying preexisting ones. 



These segments may be regarded as a series of reflex arcs, each one of 

 which may have a certain amount of physiological independence but which 

 are associated by intersegmental neurones. The latter class of intermediate 

 neurones probably effects certain groupings of various efferent neurones, fur- 

 nishing mechanisms which secure harmonious responses of groups of effectors 

 involved in certain definite reactions (e.g., limb-movements, associated eye 

 movements). These effector-associating mechanisms may be acted on di- 

 rectly (reflex) by afferent neurones or by the efferent arms of suprasegmental 

 mechanisms. 



Superadded to this segmental apparatus are the suprasegmental mechanisms 

 which develop later, the pallium being the last to be completed. These re- 

 ceive bundles from the segmental nervous system and send descending bundles 

 to the intersegmental neurones (pp. 464, 472 and 473 and Fig. 409). 



FURTHER DIFFERENTIATION OF THE NEURAL TUBE. 

 The Spinal Cord. 



From this time on, differences of structure between cord and epichordal 

 segmental brain become more marked and make it more convenient to treat 

 their later development separately. The ventral half of the cord for a con- 

 siderable period maintains its lead in development. At four weeks (Fig. 442) 

 this lead is not so pronounced as in the immediately following period. At 

 this stage it will be noticed that the lumen is narrower in the ventral part, 



