Comprehensive Functional Anatomy and Embryology of the Locomotor System

Anatomy of the Abdominal Wall and Inguinal Region

Regarding the fascia toracolumbar, it is situated between the posteroinferior and posterosuperior muscles, specifically positioned underneath the serratus muscles and sitting atop the deep musculature of the back. This fascia serves to envelope the deep (epiaxial) musculature. It is functionally related to the transversus abdominis, the internal oblique of the abdomen, and the latissimus dorsi. Anatomically, it constitutes part of the deep cervical fascia and is composed of three distinct sheets known as the anterior, media, and posterior layers.

In the abdominal region, the superficial fascia is composed of two primary layers in its inferior half: a superficial adipose layer known as the fascia de Camper and a deep membranous layer known as the fascia de Scarpa. The fascia de Scarpa continues inferiorly into the thigh, where it becomes the fascia lata, and into the perineum, where it is known as the superficial perineal fascia or fascia de Colles, which consists of aponeurotic fibers radiating posteriorly from the superficial inguinal ring.

The inguinal canal measures approximately $4\,cm$ in length and is located superior to the inguinal ligament, connecting the deep inguinal ring to the superficial inguinal ring. Its walls consist of the aponeurosis of the external oblique (anterior), the fascia transversalis and parietal peritoneum (posterior), the tendón conjunto (superior), and the inguinal ligament (inferior). In both sexes, it contains the ilioinguinal nerve and the genital branch of the genitofemoral nerve. Specifically, in males, it contains the spermatic cord (conducto espermático), and in females, it contains the round ligament of the uterus (ligamento redondo del útero). The posterior wall of the superficial inguinal ring is reinforced by the hoz inguinal or tendón conjunto, formed by the union of the internal oblique and the transversus abdominis.

Hernias in the inguinal region are categorized as direct or indirect based on their anatomical position relative to the inferior epigastric arteries. A direct inguinal hernia occurs when viscera do not pass through the inguinal canal but instead maintain a medial relationship with the inferior epigastric arteries, often involving a failure of the fascia transversalis. An indirect inguinal hernia occurs when viscera exit through the inguinal canal, typically originating at the deep inguinal ring located lateral to the lateral umbilical fold. This lateral umbilical fold is formed by the inferior epigastric vessels. The peritoneum also features a medial umbilical fold (remnants of fetal umbilical arteries) and a median umbilical fold (representative of the urachus).

Anatomy of the Head, Neck, and Trunk Musculature

Facial muscular functions include complex movements such as suction, which is primarily performed by the orbicularis oris muscle responsible for compression, contraction, and protrusion of the lips. The zygomaticus major muscle acts to pull the angle of the mouth upwards and backwards, while the levator anguli oris elevates the mouth's corner. For expressions of sadness, the depressor anguli oris is contracted. In the mental region, the mentoniano muscle elevates and displaces the lower lip anteriorly.

The muscles of mastication, which include the temporal, masseter, and medial and lateral pterygoids, are innervated by the mandibular nerve (a branch of the trigeminal nerve). The temporal muscle acts to elevate and retract (retrusión) the mandible. The lateral pterygoid is unique among these as it is responsible for the active opening (descent) and protrusion of the mouth and inserts into the neck of the mandible and the capsule of the temporomandibular joint. The masseter inserts at the lateral angle of the mandible and acts as an elevator.

The cervical region is organized into several triangles and fasciae. The posterior triangle of the neck is delimited by the sternocleidomastoideo (SCM) anteriorly, the trapezius posteriorly, and the clavicle inferiorly. Its floor contains the splenius capitis, levator scapulae, and the scalene muscles (posterior, middle, and anterior). The phrenic nerve and brachial plexus ($C5-T1$) are located here. The anterior triangle contains the suprahyoid and infrahyoid muscles. The omohyoid is an example of an infrahyoid muscle, while the stylohyoid, digastric, and mylohyoid are suprahyoid. Most infrahyoid muscles are innervated by the ansa cervicalis, whereas the geniohyoid is innervated by fibers from $C1$ traveling with the hypoglossal nerve.

The suboccipital triangle is delimited by the obliquus capitis inferior, obliquus capitis superior, and rectus capitis posterior major. The vertebral artery passes through this triangle. Regarding the lumbar region, the triangle of Grynfelt (superior lumbar triangle) is delimited by the serratus posteroinferior, internal oblique, and erector spinae, often associated with muscular hernias. The triangle of Petit (inferior lumbar triangle) is bounded by the latissimus dorsi, external oblique, and the iliac crest.

Anatomy of the Brachial Plexus and Upper Limb

The brachial plexus originates from the anterior rami of spinal nerves $C5, C6, C7, C8$, and $T1$. It is organized into trunks (superior, middle, and inferior) and fascicles (lateral, medial, and posterior). The axillary artery is surrounded by the posterior, medial, and lateral fascicles. Terminal branches include the musculocutaneous nerve (lateral fascicle), ulnar nerve (medial fascicle), and the radial and axillary nerves (posterior fascicle). The median nerve is formed by the union of the lateral and medial fascicles.

Lesions to these nerves result in specific motor and sensory deficits. A musculocutaneous nerve injury results in the loss of cutaneous sensitivity on the anterolateral face of the forearm and weakness in forearm flexion due to its innervation of the coracobrachialis, biceps brachialis, and brachialis muscles ($C5, C6, C7$). A radial nerve injury, often caused by a fracture of the humeral diaphysis or compression in the radial groove, leads to "dropped hand" (mano caída) due to inability to extend the wrist and fingers. It also provides sensory innervation to the posterior forearm and the dorsal surface of the first three fingers.

The median nerve provides innervation to most flexors of the forearm and the thenar eminence (opponens pollicis, flexor pollicis brevis, and abductor pollicis brevis) and the first two lumbricals. Carpal tunnel syndrome involves the compression of the median nerve at the flexor retinaculum, affecting the ability to use the opponens pollicis. The ulnar nerve, which passes through the canal of Guyon, innervates the adductor pollicis, all interossei muscles, and the medial two lumbricals ($3\text{rd}$ and $4\text{th}$). A lesion to the ulnar nerve results in "claw hand" (mano en garra), characterized by the atrophy of the interossei and loss of fine motor control in the fingers.

The wrist and hand feature several critical structures including the tabaquera anatómica (anatomical snuffbox), delimited by the extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus. Its floor consists of the scaphoid and trapezium bones, and the radial artery passes through it. The tendons of the hand are protected by fibrous and synovial sheaths. The extensor tendons on the dorsal side of the wrist are organized into six osteofibrous compartments: 1 (APL and EPB), 2 (ECRL and ECRB), 3 (EPL), 4 (ED and EI), 5 (EDM), and 6 (ECU).

Anatomy of the Lower Limb and Pelvis

The femoral region contains the femoral triangle (triangle of Scarpa), bounded by the inguinal ligament (base), sartorius (lateral), and adductor longus (medial). Its floor is formed by the iliopsoas and pectineus muscles. Passing through this triangle in a lateral-to-medial direction (VAN) are the femoral nerve, femoral artery, and femoral vein. The femoral canal, located medial to the femoral vein, contains the Cloquet lymph node and allows for the passage of femoral hernias. The iliopsoas is the primary flexor of the hip, while the quadriceps (innervated by the femoral nerve) is the main extensor of the knee.

In the gluteal region, the greater sciatic notch is crossed by the piriformis muscle, the superior and inferior gluteal vessels and nerves, the sciatic nerve, and the pudendal vessels and nerve. The sciatic nerve ($L4-S3$) exits through the infrapiriform space. The lesser sciatic notch is crossed by the obturator internus muscle and the pudendal vessels and nerve. A lesion to the superior gluteal nerve leads to Trendelenburg gait due to paralysis of the gluteus medius and minimus, which are critical high-level abductors. The gluteus maximus is the primary extensor of the hip, innervated by the inferior gluteal nerve.

The knee joint is reinforced by the cruciate ligaments (anterior and posterior) and collateral ligaments (tibial/medial and peroneal/lateral). The anterior cruciate ligament (ACL) is frequently injured via sudden rotations or hyperextension, tested using the "drawer test" (cajón). The medial meniscus is C-shaped and more fixed than the O-shaped lateral meniscus. The "unhappy triad" (tríada desgraciada) involves the simultaneous rupture of the ACL, medial collateral ligament, and medial meniscus. The popliteal fossa (rombo poplíteo) is a diamond-shaped space behind the knee containing the popliteal artery/vein and the division of the sciatic nerve into the tibial and common peroneal nerves.

The foot is characterized by its arches (medial longitudinal, lateral longitudinal, and transversal) and specific tendinous crossings. The decusación sural involves the crossing of the flexor digitorum longus over the tibialis posterior behind the medial malleolus. The decusación plantar occurs in the sole, where the flexor digitorum longus crosses the flexor hallucis longus. Inversion/supination injuries of the ankle most commonly affect the anterior talofibular ligament (astragaloperoneo anterior). The sural nerve provides sensory innervation to the posterolateral leg, while the saphenous nerve (from the femoral nerve) provides sensation to the medial leg and foot.

Embryology of the Musculoskeletal System

The appendicular skeleton (limbs) derives from the somatic lateral mesoderm, while the musculature of the limbs originates from the paraxial mesoderm. The lower limbs undergo a medial rotation of approximately $\pi/2$ radians or $90^{\circ}$ during the $8\text{th}$ week of gestation. Vertebrae are formed from the sclerotomes of paraxial mesoderm; errors in the migration of these cells can result in congenital scoliosis. The intervertebral disc is formed such that only the nucleus pulposus is a remnant of the embryonic notocorda.

Axial musculature is divided into epimeres and hypomeres. Epimeres give rise to the epaxial muscles (extensors and rotators of the spine, such as the iliocostalis), which are innervated by dorsal rami of spinal nerves. Hypomeres give rise to hypaxial muscles (limbs and trunk wall, such as the rectus abdominis and intercostals), innervated by ventral rami. The diaphragm has a complex origin including the septum transversum (forming the central tendon), pleuroperitoneal membranes, and myotomes from cervical levels $C3, C4, C5$. Congenital diaphragmatic hernias result from the failure of the pleuroperitoneal canals to close.

Cranial bones have varied origins: the frontal bone and zygomatic bone derive from the neural crest through membranous and endochondral ossification, respectively. The parietal bone derives from paraxial mesoderm. The base of the occipital bone develops from the paracordal cartilage, while the ethmoid develops from the precordal cartilage. The body of the sphenoid originates from the hypophyseal cartilage, and its wings develop from the optic capsule.

Questions & Discussion

Question: Why is the needle inserted above the inferior rib during an intercostal procedure? Response: The intercostal neurovascular bundle (Vein, Artery, Nerve) is located in the subcostal groove at the inferior border of the rib. To avoid damaging these structures, the needle must be introduced just above the superior border of the inferior rib.

Question: Does the median nerve innervate the adductor pollicis? Response: No. While the median nerve innervates most thenar muscles (Opponens, Abductor brevis, Flexor brevis), the adductor pollicis is strictly innervated by the deep branch of the ulnar nerve.

Question: What are the markers for the vertebral levels of the diaphragm openings? Response: The vena cava orifice is at approximately $T8$ (right side of central tendon), the esophageal hiatus is at $T10$ (formed by fibers of the right pillar), and the aortic hiatus is at $T12$ (posterior to the diaphragm, formed by the median arcuate ligament).

Question: What is the significance of the Point of Erb? Response: It is the location behind the sternocleidomastoideo muscle where the cutaneous sensory branches of the cervical plexus (great auricular, lesser occipital, transverse cervical, and supraclavicular nerves) emerge to provide sensitivity to the neck and parts of the head.