. Outlines of zoology. d four metotic. The olfactory and optic nerves are quite bythemselves and not segmental. (2) Like the spinal nerves, the cranial nerves have primarily tworoots,—a dorsal and a ventral, but the ventral roots do not join the SPINAL NERVES. 491 dorsals, which have a more superficial course and include numerou!motor fibres (correlated with the great development of visceralmusculature in the head). (3) The pre-mandibular primitive segment (I.) was probably suppliedby the oculomotor (ventral) and the ophthalmicus profundus (dorsal). The mandibular primitive segment (II.) was p
. Outlines of zoology. d four metotic. The olfactory and optic nerves are quite bythemselves and not segmental. (2) Like the spinal nerves, the cranial nerves have primarily tworoots,—a dorsal and a ventral, but the ventral roots do not join the SPINAL NERVES. 491 dorsals, which have a more superficial course and include numerou!motor fibres (correlated with the great development of visceralmusculature in the head). (3) The pre-mandibular primitive segment (I.) was probably suppliedby the oculomotor (ventral) and the ophthalmicus profundus (dorsal). The mandibular primitive segment (II.) was probably supplied bythe pathetic (ventral) and the trigeminal (dorsal). The hyoid primitive segment (III.) was probably supplied by theabducens (ventral) and the facial (dorsal). The auditory, glosso-pharyngeal, and vagus nerves have no ventral roots. Spinal nerves.—Each spinal nerve has two roots — adorsal, posterior, or sensory, and a ventral, anterior, ormotor. These arise separately and independently, but P, N. Fig. 261.—Diagrammatic section of spinal cord. ^ Posterior fissure; /.c, posterior column of whitematter; , dorsal, posterior, sensory_ or afiFerentroot; ^., ganglion ; , ventral, anterior, motor orefferent root; , compound spinal nerve withbranches: j.^., sympathetic ganglion; anteriorcolumn—the anterior fissure is exaggerated ; £:c.,ganglion cells ; £.nt., grey matter ; white matter. combine in the vicinity of the cord to form a single dorsal root exhibits at an early period a large ganglionicswelling—the spinal ganglion; the ventral root is apparentlynon-ganglionated. Moreover, the dorsal root has typicallya single origin (as in the cranial nerves), while that of theventral root is often multiple. The dorsal roots are outgrowths of a continuous ridge or crest alongthe median dorsal line of the cord. As the cord grows the nerve rootsof each side become separated. They shift sidewards and downwardsto the sides
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
