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THE NERVOUS SYSTEM. J21

which is quite extensive and divided into two portions or roots. One
root arises from the lower portion of the medulla oblongata, and the
other from the cervical portion of the spinal cord.
Fig. 154.

The Side of the Neck : 1, occipital artery ; 2, facial vein ; 3, spinal accessory nerve ; 4, facial artery ; 5,
internal jugular vein (5, hypo<,'lossal nerve; 7, coniniunicans iioni nerve; 8, lingual artery;
; 9,
pneumogastric nerve; 10, superior laryngeal nerve; 11, phrenic nerve; 12, superior thyroid
artery ; 13, sterno-cleido-niastoideus (reflected) ; 14, common carotid artery with descendens noni
nerve; lo, inner end of clavicle reflected Ki, stertio-hyoid 17, subclavian vein (cui) 18, omo-
; ; ;
hyoid; 19, subclavian artery giving off the thyroid axis and the internal mammary artery
; 20,
inferior cervical ganglion of sympathetic; 21, apex of pleura.
The Medullary or Accessor}/ Portion or Roof.—The superficial or ap-
parent origin is by four or five delicate filaments situated in the groove
between the olivary and restiform bodies on the side next the medulla
oblongata, just below the superficial origin of the pneumogastric nerve.
The Cervienl Portion or Root arises by six or eight filaments from
the lateral tract of the entire length of the cervical portion of the spinal
cord, though its superficial origin is not generally distinguishable below
the fiiurth or fifth cervical nerve. The lowest filament is generally
single the others, however, emerge from the cord in pairs. These
;
filaments ascend along the cord, increasing in size as each additional
filament is added, and extending between the ligamentum denticulatum
and the posterior roots of the spinal nerves. The first and second spinal
nerves are often connected to these filaments. After reaching the for-
amen magnum, through wliich it enters the brain-case, it curves outward
to the middle compartment of the posterior lacerated foramen, in which
it joins the medullary root. The two roots then interchange fibres with
Vol. I.—21