Manual osteopathic practitioners must understand the anatomy of the cranium (skull) bones and their joints in detail to practice craniosacral osteopathy. One of the key concepts of craniosacral osteopathy is that slight movements at the cranial joints allow for the rhythmic fluctuation of cerebrospinal fluid (CSF), which is central to overall health. Therefore, subtle movements of cranial bones are the gateways for the practitioners to assess for the ease of CSF circulation and the state of vitality of the whole body. Let's delve into the fascinating anatomy of the cranium and their movements. Brace yourself! There are a lot of information here that can bore you from time to time. If you are a healthcare practitioner, it will be worth reading through all of the information. If you have general interest and detailed anatomy is not relevant to you, skip to the summary and conclusion section and work you way backward to get more knowledge of the parts that particularly capture your attention.
Anatomy of Neurocranium and Viscerocranium.
The cranium, also known as the skull, is a complex structure composed of different bones that safeguard the brain and provide support to our facial features. Understanding the composition of these bones is essential to grasp the significance of the cranial joints in maintaining the integrity of the skull. Knowledge of the anatomical landmarks helps the practitioner perform precise techniques that influence the cerebrospinal circulation and cranial nerves that pass through various openings in the cranial bones and cranial joints.
The cranium can be divided into neurocranium and viscerocranium. The neurocranium is the part of the skull that encloses and protects the brain. It forms the upper and posterior part of the skull, as well as the base, and is sometimes referred to as the cranial vault. Its primary role is to provide a protective shell around the brain and the upper part of the spinal cord. There are 8 bones in the neurocranium and are categorized into either paired (two of each bone) or unpaired (a single bone) bones.
Unpaired neurocranium bones
Frontal Bone Situated at the front of the skull, the frontal bone plays a crucial role in protecting the frontal lobes of the brain. Its structure is notably smooth, contributing to the overall aesthetic of the face. It composes the front and top part of the orbits (eye sockets).
Ethmoid Bone Positioned between the two orbits, forming the medial walls of the orbits. It is shaped like a walnut. Together with the lesser wings of the sphenoid and the frontal bone, they also form the floor of the anterior cranial fossa. Anterior crania fossa is a shallow depression or bowl-shaped area at the front of the cranium, providing a place where part of the brain sits securely). The cribriform plate is a horizontal, sieve-like structure that is part of the ethmoid bone, located at the roof of the nasal cavity and forming part of the floor of the anterior cranial fossa. The primary function of the cribriform plate is to allow the olfactory nerves to pass from the nasal cavity to the brain. These nerves detect smells and transmit this information to the brain for processing. At the midline of the cribriform plate, there is a vertical bony ridge called the crista galli, which provides an attachment point for the falx cerebri, a membrane that separates the two hemispheres of the brain.
Sphenoid Bone Acting as a keystone in the skull, the sphenoid bone is essential for structural stability. It has wings, resembling a butterfly. These wings contributes to the interconnectedness of various skull regions. It forms the bottom and front portion of the skull, while also forming the back portion of the orbit and most of the temple region. The sphenoid forms the middle cranial fossa that situated behind the nasal cavity and beneath the brain. The sella turcica is found in the middle cranial fossa that houses the pituitary gland, a vital gland that controls many endocrine functions. The sphenoid bone contains several important foramina (openings) through which various nerves and blood vessels pass. Optic canal provides a passage for the optic nerve that transmits visual information from the eye to the brain. Ophthalmic artery also passes through the optic canal. Superior orbital fissure provides pathways for oculomotor nerve, trochlear nerve, and abducens nerve that controls eye movement and pupil constriction. Ophthalmic branch of the trigeminal nerve that transmits sensation from the forehead, scalp, and upper eyelids also passes through the superior orbital fissure. The ophthalmic veins pass through this fissure as well. The maxillary branch of the trigeminal nerve passes through foramen rotundum and relays the sensation from the midface region, including the cheeks, upper lip, and nasal cavity. Foramen ovale provides a passage for the mandibular branch of the trigeminal nerve that supplies sensation to the lower face and motor control of the muscles involved in chewing (mastication). Foramen spinosum is located in the greater wing of the sphenoid and is where the meningeal branch of the mandibular branch of the trigeminal nerve passes through. This nerve provides sensory innervation to dura mater (a meningeal layer). The middle meningeal artery also passes through the foramen spinosum. Foramen lacerum is where the internal carotid artery that supplies the brain passes through.
Occipital Bone Found at the back of the skull, the occipital bone shields the cerebellum and brainstem. Its unique opening, the foramen magnum, allows the spinal cord to connect with the brain. Together with the temporal bones, occipital bone also forms jugular foramen where the glossopharyngeal nerve, the vagus nerve, and the spinal accessory nerve exit the skull. The glossopharyngeal nerve provides sensory innervation to the posterior third of the tongue for taste and general sensation, the tonsils, and the pharynx. It also provides motor innervation to the stylopharyngeus muscle, which helps with swallowing. The vagus nerve is the longest cranial nerve that carries motor nerve for speaking and swallowing, provides sensations to throat, larynx, lung, heart, and digestive tract, and is responsible for parasympathetic regulation of heart rate, respiratory rate, and digestive functions. The spinal accessory nerve provides motor functions to trapezius and sternocleidomastoid muscles that are responsible for elevating the shoulders and turning the head. The occiptal bone also provides the hypoglossal canal through which the hypoglossal nerve exits the skull. Hypoglossal nerve provides motor functions to the tongue.
Paired neurocranium bones
Parietal Bones Positioned on each side of the skull, the parietal bones form the majority of the skull's roof. Interlocking with each other and other bones, parietal bones create a strong and protective barrier around the brain.
Temporal Bones Located at the sides and base of the skull, the temporal bones house critical structures like the inner ear that contains the organs of hearing and equilibrium. Their intricate contours accommodate various sensory organs and provide attachment points for jaw muscles. The styloid process provides the attachment sites for muscles associated with the tongue and hyoid bone. The temporal bones also form the jugular foramen by articulating with the occipital bone. The temporal bone also provides internal acoustic meatus through which the fascial and vestibulocochlear nerves pass. The facial nerve supplies the muscles of fascial expression. The vestibulocochlear nerve is crucial for hearing and balance.
The viscerocranium, also known as the facial skeleton, is the part of the skull that forms the structure of the face. It includes the bones that support and shape the face, as well as those involved in functions like breathing, eating, and sensory perception. The viscerocranium surrounds the nasal cavity, mouth, and much of the eye sockets. There are 14 bones in the viscerocranium, and they are categorized as either paired (two of each bone) or unpaired (a single bone).
Unpaired viscerocranium bones
Mandible – This is the lower jawbone, the largest and strongest bone in the face. It holds the lower teeth and forms the temporomandibular joint by articulating with the temporal bones.
Vomer – A small bone that forms part of the nasal septum, dividing the nasal cavity into two parts. Its name can from its shape, resembling a plow.
Paired viscerocranium bones
Maxillae – These bones form the upper jaw and support the upper teeth. It is part of the maxillo-palatine complex or hard palate that forms the roof of the mouth and separates the oral cavity from the nasal cavity.
Zygomatic Bones – Commonly known as the cheekbones, they form the outer and bottom parts of orbits.
Nasal Bones – These small bones form the bridge of the nose.
Lacrimal Bones – Small, fragile bones located in the medial wall of each orbit; they help form the tear duct system.
Palatine Bones – These bones contribute to the structure of the hard palate (roof of the mouth), the floor of the orbit, and the nasal cavity. They act as a spacer between the maxillae and the sphenoid.
Nasal Conchae – Also known as nasal turbinates, these bones consist of three long curved shelves of bones that protrude into the nasal cavity. These bones form part of the lateral walls of the nasal cavity and help filter and humidify the air we breathe. The superior and middle conchae articulate with each side of the ethmoid bone, while the inferior concha articulates with maxilla, lacrimal, and palatine bones. The superior concae are the smallest conchae and serve to protect the olfactory bulb (part of the central nervous system that is involved with sense of smell). The middle conchae are the second longest conchae, and act as buffers to protect the maxillary and ethmoid sinuses from pressurized nasal airflow. The inferior conchae are the longest, and are responsible for the humidification, heating, and filtering the air flowing through the nose.
The Significance of Sutures
Sutures, the fibrous joints that unite different cranial bones, are vital for both structural integrity and flexibility. These interlocking seams allow for the slight movement of bones, especially during childbirth and infancy, ensuring the skull can adapt to growth and external forces. With age, the fibrous tissue in the suture is gradually replaced by bone. For this reason, it was traditionally believed that the cranial bones have no movement. However, after many years of study and research by Dr. Sutherland and Dr. Upledger, it is now established that each of cranial bone has an inherent and involuntary motion that is permitted by sutures. The shape of the sutures allows the articulating cranial bones to have a slight and coupled motion like the cogwheels of a clock.
Cranial bones and sutures may become restricted or misaligned due to trauma, stress, or disease. Understanding the specific articulations and anatomy helps craniosacral therapists locate areas of tension or restriction. Let us delve deeper into some of the prominent cranial sutures in the neurocranium that are important for craniosacral osteopathy:
Coronal Suture Running horizontally across the skull, the coronal suture joins the frontal bone with the two parietal bones. Its distinctive shape resembles a crown, hence its name.
Frontoethmoidal Suture Joins the frontal bone and the ethmoid bone. This suture is only seen with the calvaria (upper and dome-like part of skull) removed. This suture is shaped like a horseshoe.
Lambdoid Suture Resembling the Greek letter lambda (Λ), the lambdoid suture unites the parietal and occipital bones at the back of the skull. This structure is usually continuous with the occipitomastoid suture.
Occipitomastoid Suture Connects the occipital bone and the mastoid part of the temporal bone. It is commonly continuous with the lambdoid suture.
Parietomastoid Suture Joins the parietal bone and the mastoid part of the temporal bone.
Sagittal Suture Extending from the frontal bone to the occipital bone, it joins the two parietal bones.
Sphenoethmoidal Suture Joins the sphenoid and ethmoid bone.
Sphenofrontal Suture Connects the sphenoid bone to the frontal bone
Sphenoparietal Suture Joins the sphenoid bone to the parietal bones
Sphenosquamosal Suture Connects the sphenoid bone and the squamous portion of the temporal bones. This suture is continuation of the squmous suture.
Squamous Suture Connects the parietal bones to the squamous portion of the temporal bones. The founder of cranial osteopathy, Dr. William Sutherland considered this suture as 'gill-like' suture. I am wondering why it is not named parietosquamous suture.
Intersection and Key Regions of the Sutures
Bregma is the anatomical point where the coronal suture and sagittal suture intersect.
Vertex is the most cranial part of the sagittal suture. It is also the most cranial part of the skull.
Asterion is at the junction of the occipital bone, temporal bone, and parietal bone. It is located on the lateral aspect of the skull.
Pterion is at the junction of the frontal bone, the sphenoid bone, the parietal bone, and the temporal bone. It is H shaped and also located on the lateral aspect of the skull. It is the weakest part of the skull owing to the relatively thin bone and is commonly used as a landmark for neurosurgery related to many structures within cranial cavity.
Lambda is where the sagittal suture and the lambdoid suture intersect.
Knowing these key regions of the sutures is important for the manual osteopathic practitioner. One of the critical techniques for assessing the primary respiratory rhythm involves placing the index fingers over the greater wings of the sphenoid, middle fingers over the pterion, ring fingers over the asterion, and little fingers over the lateral angle of the occiput.
Sphenobasilar Synchodrosis
The sphenobasilar synchondrosis (SBS) is the cartilaginous joint between the sphenoid bone and the basilar part of the occipital bone at the base of the skull. The movement of SBS is considered a key articulation that reflects the movement of the primary respiratory mechanism (PRM). PRM is a core concept in craniosacral therapy, describing the subtle rhythmic movements of the cranial bones, sacrum, cerebrospinal fluid (CSF), and membranes that surround the brain and spinal cord. Although SBS is not a suture, it plays a central role in creating rhythmic motion at the sutures. The SBS has subtle, rhythmic, continuous, and involuntary motion, influencing the flow of CSF and the alignment of the cranial bones.
Influence of Sphenobasilar Synchondrosis on Motion of Suture
Sphenobasilar synchondrosis (SBS) has rhythmic flexion and extension motion. During SBS flexion, the front part of the sphenoid (i.e., pterygoid processes and greater wing) moves towards the feet while the back part (i.e., basilar portion) moves towards the vertex (top of the head). This motion is coupled with the motion of the occiptital bone in which the front part (i.e., basilar portion) moves toward the top of the head and the back part (i.e., condylar and squamous portion) moves toward the feet. Such a coupled motion of sphenoid and occipital bone that occure during SBS flexion also make the vomer and ethomoid move in a same pattern as the occipital bone due to their cogwheel-like sutural connection to the sphenoid. Sphenoid, occipital bone, ethomoid, and vomer are unpaired single bones that are located in the midline of the cranium. Their movement also create motion of the paired bones due to their connection through suture. During SBS flexion, the paired bones like parietal bones, temporal bones, zygomatic bones, and maxillae externally rotate. During the SBS extension, the opposite happens to all the bones mentioned above.
Influence of Sphenobasilar Synchondrosis on Cerebrospinal fluid Circulation
As mentioned above, sphenobasialr synchondrosis (SBS) flexion and extension creates the rhythmic movement of all the bones in the cranium. SBS flexion corresponds to the timing that the cerebrospinal fluid (CSF) fills in the cranium while SBS extension corresponds to the timing in which the CSF is drained out the cranium. The timing between the SBS motion and the CSF movement makes sense from the mechanical point of view. The SBS flexion makes the cranium become wider and shorter due to the external rotation of the paired bones and upward movement of the basilar portion. This creates more suitable environment for the CSF to fill in the cranium. During the SBS extension, the opposite happens, providing the environment for facilitating the drainage of the CSF.
Summary
The cranium is divided into two major regions: the neurocranium and the viscerocranium. The neurocranium, composed of eight bones (frontal, ethmoid, sphenoid, occipital, parietal, and temporal bones), forms a protective case around the brain, while the viscerocranium consists of 14 bones that shape the face, including the mandible, maxillae, nasal, and zygomatic bones. Knowledge of the anatomical landmarks helps the practitioner perform precise techniques that influence the cerebrospinal fluid (CSF) circulation and nerves and blood vessels that pass through various openings in the cranial bones and cranial joints. The sutures between cranial bones play a crucial role in craniosacral osteopathy by allowing subtle movements that contribute to CSF flow and cranial bone alignment.
The sphenobasilar synchondrosis (SBS), a joint between the sphenoid and occipital bones, is a critical site in craniosacral therapy. This joint is central to the primary respiratory mechanism (PRM), which involves the rhythmic motion of cranial bones and membranes, influencing CSF circulation and overall cranial health. The flexion and extension movements of the SBS are associated with the expansion and contraction of the cranium, facilitating the influx and drainage of CSF, respectively.
Conclusion
A thorough understanding of the anatomy of the neurocranium and viscerocranium, along with their sutural articulations, is essential for craniosacral osteopathy. These structures not only protect vital organs like the brain and sensory organs but also influence the flow of cerebrospinal fluid and nerve and vascular function. The sphenobasilar synchondrosis, in particular, plays a pivotal role in regulating the cranial rhythm and its impact on CSF circulation. This anatomical knowledge enables osteopaths to effectively diagnose and treat restrictions or dysfunctions in cranial mobility, enhancing the body’s overall structural and functional health.
References
Diminutto, D. (2022). Involuntary Mechanism Module. Manual Osteopathic College of Canada.
Parsons, J., & Marcer, N. (2005). Osteopathy: Models for Diagnosis, Treatment and Practice. Elsevier Health Sciences.
Liem, T. (2021). Membranous structures within the cranial bowl and intraspinal space. Fascia: The Tensional Network of the Human Body-E-Book: Fascia: The Tensional Network of the Human Body-E-Book, 95.
Liem, T. (2017). Development of the cranium and an outline of the growth dynamics of cranial bones. Foundations of Morphodynamics in Osteopathy: An Integrative Approach to Cranium, Nervous System, and Emotions, 449.
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