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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
of the sacral plexus. The fourth nerve is named the nervus furcalis, from the fact that it is subdivided between the two plexuses. 1

The Lumbar Plexus 2 (plexus lumbalis) (Figs. 822, 823, 824).—The lumbar plexus is formed by the loops of communication between the anterior divisions of the first three and the greater part of the fourth lumbar nerves; the first lumbar often receives a branch from the last thoracic nerve. It is situated in the posterior part of the Psoas major, in front of the transverse processes of the lumbar vertebræ.


FIG. 822– Plan of lumbar plexus. (See enlarged image)

  The mode in which the plexus is arranged varies in different subjects. It differs from the brachial plexus in not forming an intricate interlacement, but the several nerves of distribution arise from one or more of the spinal nerves, in the following manner: the first lumbar nerve, frequently supplemented by a twig from the last thoracic, splits into an upper and lower branch; the upper and larger branch divides into the iliohypogastric and ilioinguinal nerves; the lower and smaller branch unites with a branch of the second lumbar to form the genitofemoral nerve. The remainder of the second nerve, and the third and fourth nerves, divide into ventral and dorsal divisions. The ventral division of the second unites with the ventral divisions of the third and fourth nerves to form the obturator nerve. The dorsal divisions of the second and third nerves divide into two branches, a smaller branch from each uniting to form the lateral femoral cutaneous nerve, and a larger branch from each joining with the dorsal division of the fourth nerve to form the femoral
Note 1.  In most cases the fourth lumbar is the nervus furcalis; but this arrangement is frequently departed from. The third is occasionally the lowest nerve which enters the lumbar plexus, giving at the same time some fibers to the sacral plexus, and thus forming the nervus furcalis; or both the third and fourth may be furcal nerves. When this occurs, the plexus is termed high or prefixed. More frequently the fifth nerve is divided between the lumbar and sacral plexuses, and constitutes the nervus furcalis; and when this takes place, the plexus is distinguished as a low or postfixed plexus. These variations necessarily produce corresponding modifications in the sacral plexus. [back]
Note 2.  Bardeen, Amer. Jour. Anat., 1907, vol. vi. [back]

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