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The nerve to the external sphincter ani (muscular branch of the pudendal plexus) may pass between mm. coccygeus and levator ani instead of piercing the coccygeus.
Several variations in the branches of this nerve have been reported. It may send branches to the superior rectus, medial rectus, and superior levator palpebral muscles. Branches emaninating from a small ganglion connected to the nasal nerve have been followed to the oculomotor and abducens nerves. The infratrochlear branch of the nasal nerve may be missing, in which case the areas normally supplied by this branch (skin of the upper eyelid, root of nose, conjunctiva, lacrimal caruncle and sac) receive their supply from the supratrochlear branch of the frontal nerve.
The sympathetic plexus in the region of the middle cervical ganglion may be posterior to the inferior thyroid artery or may be bifurcated, with the artery lying between the two trunks. The middle cervical ganglion may be absent, in which case the middle cardiac nerves are carried in the interganglionic chain.
See also: Median Nerve Ulnar Nerve
Some of the branches of the smaller, superior division of this nerve may arise directly from the main trunk before it divides in two. In some cases, the two divisions of the nerve are separated by a small fibrous band (pterygosphenoid ligament), which extends from the root of the external (lateral) pterygoid plate to a point on the greater wing of the sphenoid in front of the foramen spinosum. This ligament is sometimes replaced by bone, and the smaller division of the nerve passes through a special foramen beneath the greater wing of the sphenoid, just lateral to the foramen ovale.
The fourth lumbar (or furcal) nerve usually contributes to both the lumbar plexus and (through the lumbosacral trunk) the sacral plexus. Contribution to the lumbosacral trunk may come from the third or the fifth lumbar nerve. Sometimes the branch from the fourth lumbar nerve to the lumbosacral plexus is absent. In such cases, the fifth lumbar nerve supplies branches to both the lumbar and sacral plexuses. Thus, contributions to both plexuses may come from the fourth lumbar nerve, the third and fourth, the fourth and fifth or the fifth lumbar nerve alone.
The lumbosacral plexus is formed by the union of the anterior primary divisions of the lumbar, sacral, and coccygeal nerves. In about 50% of cases it receives a branch from the twelfth thoracic nerve. Its components are distributed to the lower extremity in a manner homologous and similar to the distribution of the parts of the brachial plexus to the upper limb.
See also: Sacral Ganglia
The ganglia of lumbar and upper sacral nerves may appear to be doubled, with two distinct swellings on the dorsal roots.
There are reports of several variations in the formation, course, and distribution of this nerve. The contribution to the nerve from the second lumbar nerve may be absent. Branches from the obturator to the following structures have been reported: mm. obturator internus, and pectineus, the obturator artery, the periosteum of the pelvic surface of os pubis. Both the anterior and posterior branches of the nerve may pass posterior to adductor brevis. The branch from the main trunk of the nerve to obturator externus may pass to the lateral (instead of medial) side of the obturator nerve.
In one study of 64 lumbar plexuses, 47 were formed by four spinal nerves, 14 by five, and three plexuses by six spinal nerves. In its usual form, the first three lumber nerves are entirely distributed to the lumbar plexus. The fourth lumbar is divided between the lumbar and sacral plexuses, hence it is called the furcal (meaning forked) nerve. The name furcal is also applied to any nerve or nerves that enter into the formation of both the lumbar and sacral plexuses. The proportion of the fourth lumbar contributing to the sacral plexus, however, is variable.
Although there are usually four lumbar ganglia, the number may vary from three to eight. When there are fewer than four ganglia they tend to be larger than "usual."
A minute sublingual ganglion has been described arising from the lingual nerve or submandibular ganglion (a ganglion of the facial nerve carried by the lingual nerve) supplying the sublingual gland. This nerve may pierce the lateral pterygoid rather than passing between the two pterygoid muscles. It occasionally provides motor branches to the medial and lateral pterygoids and to the palatoglossus muscles.
The nerve may be absent (7% on the right side, 2% on the left, and 2% bilaterally). It is usually formed from the tenth and eleventh thoracic ganglia. The nerve arises symmetrically in 35% of bodies. "Normal" (usual) nerves (T10-T11) occurred in 14% of subjects; only 1% of these were symmetric bilaterally. The greater and lesser nerves may be carried in the same trunk.
This nerve may be absent (11% on the right side and 4% on the left side). The splanchnicus imus usually arises from the twelfth thoracic ganglion. It may be absent bilaterally in 2% of bodies. When absent, the lesser splanchnic supplies the renal plexus. The nerve arises symmetrically in 79% of bodies. "Normal" nerves were found in 92% of bodies and were symmetric in 77%.
See Also: Tibial and Sural Nerves
The first and second lumbricals may receive branches from both the lateral and medial plantars. The branch of the lateral nerve to the second lumbrical courses forward, beneath the transversus (adductor hallucis), then turns backward over the transversus (adductor hallucis) to reach the lumbrical muscle. Cruveilhier described a branch of the lateral plantar that pierced the transversus to reach the third lumbrical. The lateral nerve rarely provides a branch to the lateral head of flexor hallucis brevis.
This nerve may arise from the femoral nerve or as an independent branch of the lumbar plexus (L2,L3). Several variations in the formation, course, and branches of this nerve have been reported. The nerve may be absent on one side and may be replaced by a branch of the anterior femoral cutaneous nerve or by the ilioinguinal nerve. The lateral cutaneous is sometimes associated with the anterior femoral cutaneous nerve until it passes distal to the inguinal ligament. The posterior branch of the lateral cutaneous nerve may be replaced by a branch of the genitofemoral nerve.
Purely sensory. This nerve may appear to be derived from the trochlear nerve. However, the probable source in such cases is the ophthalamic nerve through its communicating branch to the trochlear in the cavernous sinus. The lacrimal nerve may be small at its origin, increasing in size later in its course by addition of fibers derived from the temporal branch of the maxillary division of the trigeminal nerve. The lacrimal nerve may be absent and replaced by the temporal branch of the maxillary division of the trigeminal nerve.
Instead of piercing the thyrohyoid membrane to reach the larynx, this nerve may pass through a small foramen in the thyroid cartilage. The arytenoid muscle receives its primary innervation from the internal laryngeal nerve and occasionally receives an additional supply from the recurrent laryngeal nerve.