SPINAL NERVES. 491 



dorsals, which have a more superficial course and include numerous 

 motor fibres (correlated with the great development of visceral 

 musculature in the head). 



(3) The pre-mandibular primitive segment (I.) was probably supplied 

 by the oculomotor (ventral) and the ophthalmicus profundus (dorsal). 



The mandibular primitive segment (II.) was probably supplied by 

 the pathetic (ventral) and the trigeminal (dorsal). 



The hyoid primitive segment (III.) was probably supplied by the 

 abducens (ventral) and the facial (dorsal). The auditory, glosso- 

 pharyngeal, and vagus nerves have no ventral roots. 



Spinal nerves. Each spinal nerve has two roots a 

 dorsal, posterior, or sensory, and a ventral, anterior, or 

 motor. These arise separately and independently, but 



FIG. 261. Diagrammatic section of spinal cord. 



/>._/C, Posterior fissure ; p.c. y posterior column of white 

 matter; d.p.s., dorsal, posterior, sensory or afferent 

 root ; ., ganglion ; v.a.m., ventral, anterior, motor or 

 efferent root ; c.n., compound spinal nerve with 

 branches; s.g., sympathetic ganglion; a.c., anterior 

 column the anterior fissure is exaggerated ; g.c., 

 ganglion cells ; g.m.> grey matter ; w.m., white matter. 



combine in the vicinity of the cord to form a single nerve. 

 The dorsal root exhibits at an early period a large ganglionic 

 swelling the spinal ganglion ; the ventral root is apparently 

 non-ganglionated. Moreover, the dorsal root has typically 

 a single origin (as in the cranial nerves), while that of the 

 ventral root is often multiple. 



The dorsal roots are outgrowths ol a continuous ridge or crest along 

 the median dorsal line of the cord. As the cord grows the nerve roots 

 of each side become separated. They shift sidewards and downwards 

 to the sides of the cord. The ventral roots are later in arising ; they 

 spring as outgrowths from the latero-ventral angle of the cord. 



According to most authorities, the sympathetic ganglia are offshoots 

 from the same rudiment as that from which the dorsal ganglia arise, 



