Chapter 3 – Bones of the Human Body The skeleton of the head, or skull, is divided into the cranial or main skull and the facial or splanchnic skull. The bones of the cranial skull are eight: the frontal, parietal, sphenoid, ethmoid, and temporal, which are paired (one on the left and one on the right).
Frontal Bone The frontal bone is located on the anterior surface of the skull, forming the forehead.
Temporal Bone The two temporal bones are located on the upper and lateral surface of the skull.
Parietal Bone The parietal bone is located on the posterior surface of the skull.
Occipital Bone The two occipital bones are located on the lateral surface of the skull. The external auditory meatus, where we hear from, is located in the temporal bone.
Wedge-Shaped Bone The wedge-shaped bone is located at the base of the cranial skull. Ethmoid Bone The ethmoid bone is a small bone located in the roof of the nasal cavity.
The bones of the skull are connected to each other at points called cranial sutures. Babies are born with more than 300 bones, which fuse together as they develop.
[The skull of an embryo is different from that of a newborn. The mature skull can be considered a single bone whose main purpose is to protect the brain. The skull of a newborn consists of seven bones, which are separated by openings called sutures. This serves two important functions. Firstly, a skull made up of several bone plates is flexible enough to allow the head to pass through the birth canal during delivery. Secondly, it allows for easy expansion of the skull during the first two years of life, when the brain develops at a rapid pace. After just two years, the brain has reached 75% of its full volume!]There are two areas, one at the top of the head and one near the back, where three seams meet in such a way as to create larger openings called “fontanelles”. The smallest of these is located at the back of the skull, hence called the posterior fontanelle, and usually closes when the bones fuse (a process that is completed by the 3rd month). The larger fontanelle, the anterior fontanelle, usually closes between the 6th month and 2nd year.
The sagittal suture connects the two parietal bones together. The coronal suture connects the two parietal bones to the frontal bone. The lambdoid suture connects the two parietal bones to the occipital bone.
The main purpose of the cranial bones is to protect the brain from injuries. The cranial bones consist of two compact plates with a thin, sparsely populated area in between. In some areas, the two plates are separated, creating spaces filled with air called sinuses. When bacteria develop in the sinuses, inflammations occur, with the most common being sinusitis.
Bones of the Head – Cranial and Facial Skeleton
The skull, or cranium, is divided into the cranial or neurocranium and the facial or viscerocranium. The bones of the viscerocranium are fourteen, seven bones that form the nasal capsule and seven bones that form the two maxillae (upper and lower). The bones of the nasal capsule are the two nasal conchae, the two nasal bones, the two lacrimal bones, and the vomer. The bones of the maxillae are the two upper maxillae, the two zygomatic bones, the two palatine bones, and the mandible.
Nasal Conchae The nasal conchae are located in the interior of the nasal cavity.
Nasal Bones The two nasal bones are located in the bridge of the nose and are connected to each other.
Lacrimal Bones The two lacrimal bones are located in the inner wall of the orbital cavity.
Vomer The vomer participates in the formation of the nasal septum.
Upper Maxilla The two bones of each upper maxilla participate in the formation of the orbital cavity, the nasal cavity, and the oral cavity.
Zygomatic Bone The two zygomatic bones form the cheeks.
Palatine Bones The two palatine bones are located in the roof of the oral cavity and participate in the formation of the nasal cavity and the oral cavity.
Supraorbital Bones (not depicted) The supraorbital bones are located in the roof of the oral cavity and participate in the formation of the nasal cavity and the cavity of the mouth. Mandible The mandible is the only bone of the skull that exhibits mobility.
Bones of the Thorax The skeleton of the thorax consists of the 12 thoracic vertebrae, the sternum, and 12 pairs of ribs that extend from the spinal column to the sternum. [There are cases of a 13th rib and/or additional bones present from birth in some exceptions.] Sternum The sternum has a sword-like shape, is located in front of and opposite the thoracic part of the spinal column and consists of the handle, the body, and the xiphoid process. At the point of union of the handle with the body of the sternum, the sternal or Louis angle is formed, which can be easily palpated as a small bump. The base of the sternum’s handle displays the manubriosternal or jugular notch.
Ribs
The ribs are 12 pairs (right and left) and are joined at one end to the 12 thoracic vertebrae, and at the other end directly or indirectly to the sternum, forming the thoracic cavity, which protects the lungs, heart, and major blood vessels. The anterior end of the first seven ribs is directly joined to the sternum with costal cartilage. These ribs are called true ribs. The anterior end of the eighth, ninth, and tenth ribs is indirectly joined to the sternum with the costal arch. These ribs are called false ribs. The anterior end of the eleventh and twelfth ribs ends freely without joining the sternum. These ribs are called floating or vertebral ribs.
Spinal column The spinal column is located in the middle and back plane of the body, constituting the main skeletal axis of the body. It has five divisions: cervical, thoracic, lumbar, sacral, and coccygeal, and each division consists of a different number of vertebrae. When observed laterally, the spinal column presents four curvatures or arcs: the cervical, thoracic, lumbar, and sacrococcygeal. It is composed of thirty-three to thirty-four vertebrae, which are stacked on top of each other and separated by intervertebral discs. Depending on the division of the spinal column, the vertebrae are categorized as seven cervical, twelve thoracic, five lumbar, five sacral, and four to five coccygeal. The cervical, thoracic, and lumbar vertebrae are called true vertebrae, while the sacral and coccygeal vertebrae are fused together and form single bones, the sacrum and coccyx, respectively, and are called false vertebrae.
The spinal column, when observed from the side, as we mentioned, displays four curves: the cervical, thoracic, lumbar, and sacrococcygeal. Excessive development of the thoracic curve results in kyphosis, while excessive development of the lumbar curve results in lordosis. Temporary lordosis is observed in pregnant women during the last months of pregnancy. Towards the end of childhood and the beginning of adolescence, when the spinal column is observed from the front, it displays three lateral curves: the cervical, thoracic, and lumbar. Excessive development of these curves, particularly the thoracic, results in scoliosis.
Spinal Column – Vertebrae The spinal column consists of thirty-three to thirty-four vertebrae, which are stacked on top of one another and separated by intervertebral discs. Each genuine vertebra displays the body, arches, processes, and spinal foramen. The bodies of the vertebrae are stacked on top of one another, and the intervertebral disc is interposed between them, while the intervertebral foramina are located between the vertebrae, through which the spinal nerves pass. The vertebral arch is located behind the body, and the spinal foramen is created between them. The spinal foramina of the vertebrae form the spinal canal in which the spinal cord is protected.
Each intervertebral disc is composed externally of the fibrous ring and internally of the gelatinous nucleus. Excessive pressure on the intervertebral disc can cause a rupture of the fibrous ring and protrusion of the gelatinous nucleus, which is called a herniated disc. This herniation often protrudes into the spinal canal and causes sciatica or lumbago due to pressure on the spinal nerves or spinal cord. [Diseases of intervertebral discs: protrusion → prolapse → herniation → herniation with descent or ascent → stenosis, degeneration → complete degeneration, disappearance] Herniated discs, as well as other pathological conditions, can only be clearly distinguished on axial and magnetic resonance imaging.
Pelvic Bones The pelvis is formed by the two anonymous bones that are connected in front at the pubic symphysis and at the back with the sacrum. Sacrum Bone The sacred vertebrae are joined together and form a single bone, the sacrum bone. Anonymous Bone Each anonymous bone consists of three partially fused bones: the ilium, ischium, and pubis. The two anonymous bones are connected to each other in front at the pubic symphysis and at the back with the sacrum bone, forming the pelvis. Acetabulum It is the point where the pelvis connects to the femur bone, forming the hip joint.
Sacropelvic Joint The sacropelvic joint is the most complex joint of the human body, which has a high degree of stability at the expense of mobility, which is essential for walking and standing upright. The shape and dimensions of the pelvis differ between men and women. The shape and dimensions of the female pelvis are of great importance in obstetrics, as the pelvis forms the bony canal through which the fetus passes during delivery.
Bones of the Upper Limb The skeleton of the upper limb consists of the scapula, clavicle, humerus, radius, ulna, the bones of the wrist, metacarpal bones, and phalanges of the fingers.
Scapula The scapula is a wide and thin triangular-shaped bone located on the posterior surface of the thoracic wall and extends from the second to the seventh rib. At the lateral angle of the scapula is the glenoid cavity, which articulates with the humerus bone.
Clavicle The clavicle is an elongated bone that has an S shape and is located above the first rib. At one end, the clavicle is connected to the sternum and at the other end, it is connected to the scapula.
Humerus Bone The humerus bone has three parts: the head, the shaft, and the lower end. The upper end of the humerus bone has the head of the bone and is connected to the scapula, forming the shoulder joint. The shaft of the humerus bone has a cylindrical shape upwards and a prism shape downwards. The lower end of the humerus bone has two articular surfaces, the trochlea facing inward and the capitulum facing outward, which are connected to the ulna and radius bones respectively, forming the elbow joint.
Olecranon The olecranon consists of three parts: the body, the upper end, and the lower end. The upper end of the olecranon is connected to the ulna bone and the head of the radius, participating in the formation of the elbow joint. The body of the olecranon has a prism shape upwards and a cylindrical shape downwards. The lower end of the olecranon ends in the head of the ulna, which is connected to the bones of the wrist and the radius, participating in the formation of the wrist joint.
Radius The radius has a smaller length than the olecranon and consists of three parts: the body, the upper end, and the lower end. The upper end of the radius shows the head of the radius, which is connected to the ulna bone and the olecranon, participating in the formation of the elbow joint. The body of the radius has a prism shape. The lower end of the radius is connected to the bones of the wrist and the olecranon, participating in the formation of the wrist joint.
Bones of the Wrist The bones of the wrist are the scaphoid, lunate, triquetral, pisiform, greater multangular, lesser multangular, capitate, and hamate. They are arranged in two rows.
Metacarpal Bones There are five metacarpal bones, one corresponding to each finger.
Phalanges Each finger has three phalanges except for the thumb, which has two. The phalanges are named from top to bottom the proximal, middle, and distal or ungual phalanx.
Bones of the Lower Extremity The skeleton of the lower extremity is composed of two unnamed bones, the femur, the patella, the tibia, the fibula, the tarsal bones, the metatarsals, and the phalanges. Femur Bone The femur bone is the strongest and largest bone in the body, and the height of a person depends on its length. The direction of the femur bone is oblique from upper and outer to lower and inner. The femur bone appears in three parts: the body, the upper end, and the lower end. The upper end shows the head of the femur bone and is connected to the acetabulum forming the hip joint.
Patella
The patella is the bone that covers the knee joint. The patella is located within the quadriceps tendon, as it passes over the knee joint to attach to the tibia. The patella acts as a pulley to help the tendon not to be cut. Bones that form within tendons are called sesamoids, and the patella is the largest sesamoid bone in the body. It has a triangular shape. Its apex points downward and serves for the attachment of the patellar ligament, which connects the patella to the tibia. Its base is broad and rough and serves as the attachment site for the quadriceps tendon. Its posterior surface articulates with the femur and bears two facets, one medial and one lateral, with a smooth bony ridge between them.
The tibia is the largest and innermost bone of the lower leg, below the knee, and it bears the weight of the body. It is the second-largest bone in the human body. It is the only bone of the tibia that participates in the knee joint. The upper part of the tibia is broad and has two condyles. The upper surfaces of the condyles are articular. Beneath the condyles is the tibial tuberosity or elevation. The diaphysis of the tibia is triangular. Its outer surface is connected to the fibula through the interosseous membrane. The lower end of the tibia has a bony prominence, the medial malleolus.
The fibula is the outer bone of the lower leg. Unlike the tibia, it does not bear the weight of the body and therefore is thinner and does not participate in the knee joint. It has a small head, a narrow neck, and a thin shaft that ends downward at the lateral malleolus.
Bones of the Tarsus The tarsus consists of seven bones. The seven bones of the tarsus are divided into a central and a peripheral group, with an intermediate bone between the two groups on the inside of the foot. The two most massive bones of the tarsus, the astragalus, which is the highest bone in the foot, and the calcaneus, which is the largest bone of the tarsus located beneath the astragalus, are in the central group. The astragalus articulates with the tibia and fibula, the tarsometatarsal joint, and the heel bone. The intermediate bone is the navicular. The peripheral group includes the cuboid and the three wedge-shaped bones, the medial, intermediate, and lateral. The bones are connected to each other by joints and ligaments.
Metatarsal Bones There are five metatarsal bones, one for each toe.
Phalanges Each toe has three phalanges except for the big toe, which has two, just like in the hand. The phalanges are named first, second or middle, and third or distal phalanx, from top to bottom.