Canal (anatomy)

In anatomy, a canal is a tubular passage that connects two regions of the body and transmits vessels, nerves, lymphatics or a mobile organ.

The term is used at every scale, from the carotid canal inside the petrous temporal bone to the microscopic canaliculi that carry osteocyte processes through bone matrix.

Canals are defined by their walls and by what they transmit, and their clinical importance usually follows from the fact that their contents have little room to move.

Definition

• A canal is a confined conduit with an opening at each end, and its lumen is the space through which its contents pass.
• Bony canals are tunnels through compact or spongy bone, and their walls are lined by periosteum or by the dura mater where the canal opens onto the cranial cavity.
• Soft-tissue canals are bounded by fascia, muscle or aponeurosis, and they collapse when the surrounding tone is lost.
• A canal differs from a duct because a duct is usually a secretory or excretory tube of a gland, whereas a canal is a passage through or between body regions.
• A canal differs from a foramen because a foramen is a shorter opening through bone, although the two words overlap when a foramen has a substantial tunnel-like depth.
• A canal differs from a fissure because a fissure is a cleft between adjacent parts, whereas a canal is a true conduit with a continuous wall.
• Terminologia Anatomica reserves canalis for named tunnels and uses the adjective canalis in compound names, so the Latin form is more precise than the English word.

Cranial region

• The carotid canal runs through the petrous part of the temporal bone and transmits the internal carotid artery together with a venous plexus and sympathetic fibres.
• The facial canal follows a tortuous course through the temporal bone and carries the facial nerve from the internal acoustic meatus to the stylomastoid foramen.
• The optic canal lies between the lesser wing of the sphenoid and the body of the sphenoid and transmits the optic nerve and the ophthalmic artery.
• The mandibular canal runs within the mandible and carries the inferior alveolar nerve and vessels to the mental foramen, where the mental nerve emerges.
• The pterygoid canal, also called the Vidian canal, transmits the nerve of the pterygoid canal and its accompanying vessels toward the pterygopalatine fossa.
• The incisive canals open on the hard palate and transmit the nasopalatine nerves and the greater palatine vessels between the nasal and oral cavities.
• The infraorbital canal continues the infraorbital groove and transmits the infraorbital nerve and vessels to the face.
• The greater palatine canal descends from the pterygopalatine fossa and delivers the greater palatine nerve and vessels to the hard palate.

Abdominal and pelvic regions

• The inguinal canal is an oblique passage through the lower abdominal wall that transmits the spermatic cord in men and the round ligament of the uterus in women.
• The femoral canal is the medial compartment of the femoral sheath, and it contains lymph nodes and loose connective tissue rather than a major vessel.
• The obturator canal is a gap between the obturator membrane and the superior pubic ramus that transmits the obturator nerve and vessels.
• The anal canal is the terminal part of the large intestine, extending from the rectal ampulla to the anus and guarded by the internal and external anal sphincters.
• The cervical canal connects the uterine cavity to the vagina and is lined by endocervical mucosa that changes with the menstrual cycle.
• The pudendal canal, or Alcock canal, is a fascial tunnel on the lateral wall of the ischioanal fossa that carries the pudendal nerve and internal pudendal vessels.

Upper and lower limbs

• The carpal canal is the space between the flexor retinaculum and the carpal bones, and it transmits the median nerve and the flexor tendons.
• The ulnar canal, or Guyon canal, passes between the pisiform and the hook of the hamate and carries the ulnar nerve and vessels into the hand.
• The suprascapular canal is a short bony and ligamentous passage at the suprascapular notch that transmits the suprascapular nerve.
• The adductor canal is an aponeurotic tunnel in the middle third of the thigh that carries the femoral artery, femoral vein and saphenous nerve toward the popliteal fossa.
• The radial canal, more often called the radial tunnel, is the passage beneath the supinator and related muscles through which the deep branch of the radial nerve runs.
• The tarsal canal is a space between the talus and calcaneus that contains the artery of the tarsal canal and is important in the blood supply of the talus.

Development

• Bony canals form when a growing nerve or vessel becomes enclosed by advancing intramembranous or endochondral ossification, a process called secondary enclosure.
• The size of a bony canal reflects the diameter of its contents at the time of ossification, which is why canals are smaller in adults than in fetuses for the same nerve.
• Soft-tissue canals form by the rearrangement of fascial planes and muscle bellies during limb rotation and septation, not by the excavation of a pre-existing space.
• The inguinal canal shortens and rotates as the pelvis widens and the testes descend, which explains its oblique course in the adult.
• Persistent fetal canals, such as a patent processus vaginalis or a patent ductus arteriosus, are common developmental anomalies with distinct clinical consequences.

Clinical significance

• Carpal tunnel syndrome results from compression of the median nerve within the carpal canal and produces nocturnal numbness in the thumb, index and middle fingers.
• Inguinal hernia occurs when abdominal contents enter the inguinal canal, and the canal's oblique course is one reason direct and indirect hernias behave differently.
• Femoral hernia passes through the femoral canal and is more common in women, with a higher risk of strangulation because the canal is narrow and rigid.
• Obturator hernia descends through the obturator canal and can present with Howship-Romberg pain along the medial thigh due to nerve compression.
• Cervical canal stenosis or obstruction can cause infertility and haematocolpos, and the canal is routinely assessed during hysteroscopy and embryo transfer.
• Canals are also targets for nerve blockade, as in the adductor canal block for knee analgesia and the pudendal canal block for perineal pain.

Comparative anatomy

• Bony canals are widespread among tetrapods, and the carotid canal is one of the features used to infer the position of the internal carotid artery in fossil reptiles.
• The femoral canal is proportionally shorter in quadrupeds, where the femoral sheath is compressed by a more horizontal hip posture.
• The inguinal canal of mammals is homologous in its basic plan, but its length and the presence of a processus vaginalis vary with testicular descent.
• In birds the equivalent passage for the ovary and oviduct is the pelvic canal, and the bony canals of the wing are reduced where flight muscles replace heavy bone.
• Fish and amphibians lack several mammalian canals because their visceral anatomy does not require the same enclosed routes for gonads and urinary ducts.

History

• The Latin canalis originally meant a pipe or groove, and classical anatomists applied it to any long narrow passage before modern terminology restricted the word.
• Vesalius and his successors described the cranial canals in the sixteenth century, but many were named only after their contents were traced in the nineteenth century.
• The eponymous Alcock canal and Guyon canal honour nineteenth-century clinicians, while Terminologia Anatomica now prefers descriptive names such as canalis pudendalis.
• Terminologia Anatomica, first published in 1998 and revised in 2019, lists canalis as a preferred term and discourages the use of older eponyms in formal nomenclature.

Research and controversies

• High-resolution computed tomography and three-dimensional reconstruction have shown that bony canals are more variable in shape than textbook drawings suggest.
• Measurements of the carpal canal by ultrasound and magnetic resonance imaging differ systematically, so studies must state which modality and which landmark they used.
• Whether the adductor canal ends at the adductor hiatus or continues into the popliteal fossa remains debated, and the answer changes the target of a nerve block.
• The term canal is sometimes applied to spaces that are not true conduits, such as the canal of Schlemm, and this looseness of usage complicates anatomical databases.