There are six pairs of =lumbar nerves= (Nn. lumbales) in the horse, the
last of which emerge between the last lumbar vertebra and the sacrum.
The anterior two or three are about the same size as the thoracic
nerves, but the others are much larger.
[Illustration:
FIG. 523.—SUPERFICIAL NERVES OF NECK AND TRUNK OF HORSE. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
Their =dorsal branches= are small in comparison with the ventral ones.
They are distributed to the muscles and skin of the loins and croup in a
fashion similar to those of the thoracic nerves.
The =ventral branches= are connected with the sympathetic by small rami
communicantes, and give branches to the sublumbar muscles. Those of the
first two nerves are arranged in a manner analogous to the corresponding
branch of the last thoracic nerve.
The ventral branch of the =first lumbar nerve= is termed the
=ilio-hypogastric nerve= (N. iliohypogastricus). It passes outward
between the quadratus lumborum and the psoas major, and divides at the
outer border of the latter into an anterior or superficial and a
posterior or deep branch. The =anterior= or =superficial branch= passes
over the upper edge of the internal oblique, descends between that
muscle and the external oblique, perforates the latter, and runs
downward and backward and ramifies under the skin of the posterior part
of the flank and the outer surface of the thigh. It gives branches to
the transversus and obliquus externus abdominis. The =posterior= or
=deep branch= is smaller; it runs downward and backward beneath the
peritoneum to the outer border of the rectus abdominis, gives branches
to the internal oblique, and terminates in the rectus abdominis.
[Illustration:
FIG. 524.—LUMBAR NERVES OF HORSE, VENTRAL VIEW. (After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]
The ventral branch of the =second lumbar nerve= is connected by an
anastomotic branch with that of the third nerve. It gives off a large
branch to the psoas muscle and an inguinal branch, and is continued as
the ilio-inguinal nerve. The =inguinal branch= (N. spermaticus externus)
runs backward in the substance of the psoas minor, emerges a short
distance in front of the circumflex iliac vessels, and runs backward and
downward under the peritoneum to the internal inguinal ring. It gives
branches to the internal oblique muscle and descends in the inguinal
canal along the outer border of the cremaster (to which it detaches
filaments) and ends subcutaneously in the scrotum and prepuce in the
male, the mammary gland in the female. The =ilio-inguinal nerve= (N.
ilioinguinalis) may be regarded as the continuation of the ventral
branch. It runs parallel with the ilio-hypogastric nerve and has a
similar arrangement. Its =anterior= or =superficial branch= perforates
the external oblique muscle a little in front of the point of the hip,
runs downward on the front of the thigh and the outer surface of the
stifle, and gives off cutaneous branches. The =posterior= or =deep
branch= runs behind and parallel with that of the iliohypogastricus,
detaches branches to the internal oblique muscle, and descends the
inguinal canal with the inguinal branch, to be distributed to the
external genital organs and the surrounding skin in the inguinal region.
The ventral branch of the =third lumbar nerve= is connected by a small
anastomotic branch with the second nerve and furnishes a root of the
lumbo-sacral plexus. It gives off a branch to the psoas muscles, an
inguinal branch, and is continued as the external cutaneous nerve. The
=inguinal branch= (N. spermaticus externus) passes backward in the
substance of the psoas minor, from which it emerges under cover of or
near the circumflex iliac vessels. It then runs external to and parallel
with the external iliac artery and descends in the inner part of the
inguinal canal. It emerges at the external ring with the external pudic
artery and ramifies in the external genital organs and the skin of the
inguinal region. The =external cutaneous nerve= of the thigh (N.
cutaneus femoris lateralis) runs backward in the substance of the psoas
muscles and emerges at the outer border of the psoas minor. It then
passes outward and backward on the iliac fascia and accompanies the
posterior branch of the circumflex iliac artery. With this vessel it
perforates the abdominal wall by passing between the external oblique
and the iliacus a short distance below the point of the hip, and
descends on the inner face of the tensor fasciæ latæ (near its anterior
border) and ramifies subcutaneously in the region of the stifle.
[Illustration:
FIG. 525.—SUPERFICIAL NERVES OF PELVIC LIMB AND POSTERIOR PART OF
TRUNK OF HORSE.
_a_, Cutaneous branches of sixteenth and seventeenth thoracic nerves;
_b_, cutaneous branches of lumbar nerves; _c_, cutaneous branches of
sacral nerves; _d_, cutaneous branches of coccygeal nerves; _e_,
_f_, _g_, cutaneous branches of last intercostal, ilio-hypogastric,
and ilio-inguinal nerves; _g′_, end of external cutaneous nerve of
thigh; _h_, posterior cutaneous nerve of thigh; _i_, _i_, cutaneous
branches of great sciatic nerve; _k_, posterior cutaneous nerve of
the leg; _l_, superficial peroneal nerve; _m_, terminal part of deep
peroneal nerve; _n_, external plantar nerve; _1_, obliquus abdominis
externus; _2_, tensor fasciæ latæ; _3_, gluteus superficialis; _4_,
biceps femoris; _5_, semitendinosus; _6_, anterior extensor; _7_,
lateral extensor; _8_, flexor tendons; _9_, great metatarsal artery.
(After Ellenberger, in Leisering’s Atlas.)
]
The origin and disposition of some of the foregoing nerves are
variable. In some cases the ilio-inguinal nerve ends in the psoas
major, and appears then to be absent. The mode of formation of the
inguinal nerves is very inconstant. The inner nerve may arise with one
of the outer ones from a common trunk, or they may anastomose. The
inguinal branch of the ilio-inguinal nerve often receives a twig from
the ilio-hypogastric nerve.
The ventral branches of the =fourth=, =fifth=, and =sixth lumbar= nerves
concur in the formation of the lumbo-sacral plexus.