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 
Fic. 261.—Diagrammatic section of spinal cord. 
@4-, Posterior fissure; g.c., posterior column of white 
matter; @.f.s., dorsal, posterior, sensory or afferent 
root ; g-, ganglion ; ¥.a.12., ventral, anterior, motor or 
efferent root; c.#., compound spinal nerve with 
branches; s.g., sympathetic ganglion; @.c., anterior 
column—the anterior fissure is exaggerated; g.c., 
ganglion cells ; g.22., grey matter ; z.7., 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 of 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, 
