Head and Neck



Head and Neck



1 Introduction


The head and neck area offers a unique challenge for students because of the density of small neurovascular structures; the complexity of its bony features, especially the skull; and the compactness of its anatomy. The head protects the brain, participates in communication and expresses our emotions, and houses the special senses (sight, sound, balance, smell, and taste). The neck connects the head to the thorax and is the conduit for visceral structures passing cranially or caudally within tightly partitioned fascial sleeves.


The anatomy of the head is best understood if you view it as a series of interconnected compartments, which include the following:



The anatomy of the neck is composed of a series of concentric-like compartments that provide a conduit for structures passing to the head or thorax, as follows:




2 Surface Anatomy


The key surface features of the head and neck and include the following (Fig. 8-1):


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FIGURE 8-1 Key Surface Anatomy Landmarks of the Head and Neck. (From Atlas of human anatomy, ed 6, Plate 1.)



3 Skull


The skull is composed of 22 bones (see Chapter 1). Eight of these bones form the cranium (neurocranium, which contains the brain and meninges), and 14 of these form the face (viscerocranium). There are seven associated bones: the auditory ossicles (three in each middle ear) and the unpaired hyoid bone (Fig. 8-2 and Table 8-1). Using your atlas and dry bone specimens, note the complexity of the maxillary, temporal, and sphenoid bones. These bones are in close association with many of the cranial nerves and encase portions of many of our special senses—balance, hearing, smell, sight, and even taste—as the maxillae form a portion of the oral cavity.



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FIGURE 8-2 Anterior and Lateral Views of the Skull. (From Atlas of human anatomy, ed 6, Plate 4.)

Other features of the skull are noted as we review each region of the head. However, general external features include the following (Figs. 8-2 and 8-3):


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FIGURE 8-3 Sagittal Sections of the Skull. (From Atlas of human anatomy, ed 6, Plates 6 and 8.)



Cranial Fossae


The cranial base is the floor of the neurocranium, which supports the brain, and is divided into the following three cranial fossae: (Fig. 8-4):


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FIGURE 8-4 Superior Aspect of Cranial Base (Cranial Fossae). (From Atlas of human anatomy, ed 6, Plate 13.)


Each fossa has numerous foramina (openings) for structures to pass in or out of the neurocranium.



Clinical Focus 8-1   Skull Fractures


Skull fractures may be classified as follows:



Any fracture that communicates with a lacerated scalp, a paranasal sinus, or the middle ear is termed a compound fracture. Compound depressed fractures must be treated surgically.


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Clinical Focus 8-2   Zygomatic Fractures


Trauma to the zygomatic bone (cheekbone) can disrupt the zygomatic complex and its articulations with the frontal, maxillary, temporal, sphenoid, and palatine bones. Often, fractures involve suture lines with the frontal and maxillary bones, resulting in displacement inferiorly, medially, and posteriorly. The typical clinical presentation is illustrated. Ipsilateral ocular and visual changes may include diplopia (an upper outer gaze) and hyphema (blood in the anterior chamber of the eye), which requires immediate clinical attention.


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Clinical Focus 8-3   Midface Fractures


Midface fractures (of the maxilla, naso-orbital complex, and zygomatic bones) were classified by Le Fort as follows:



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4 Brain


Meninges


The brain and spinal cord are surrounded by three membranous connective tissue layers called the meninges, which include the following (Fig. 8-5):


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FIGURE 8-5 Central Nervous System Meninges, Cerebrospinal Fluid Circulation, and Arachnoid Granulations. (From Atlas of human anatomy, ed 6, Plate 110.)


The cranial dura is distinguished from the dura mater covering the spinal cord by its two layers. An outer periosteal layer is attached to the inner aspect of the cranium and is supplied by the meningeal arteries, which lie on its surface between it and the bony skull. Imprints of these meningeal artery branches can be seen as depressions on the inner table of bone. This periosteal dura is continuous with the periosteum on the outer surface of the skull at the foramen magnum and where other intracranial foramina open onto the outer skull surface. The inner dural layer is termed the meningeal layer and is in close contact with the underlying arachnoid mater and is continuous with the spinal dura at the level of the foramen magnum.


The dura mater is richly innervated by meningeal sensory branches of the trigeminal nerve (fifth cranial nerve, CN V); the vagus nerve (CN X), specifically to the posterior cranial fossa; and the upper cervical nerves. A portion of the dura in the posterior cranial fossa also may receive some innervation from the glossopharyngeal nerve (CN IX) and hypoglossal nerve (CN XII). The arachnoid and pia mater lack sensory innervation. The periosteal dura and meningeal dura separate to form thick connective tissue folds or layers that separate various brain regions and lobes (Figs. 8-5, 8-6, and 8-7):


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FIGURE 8-6 Dural Projections.

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FIGURE 8-7 Dural Venous Sinuses. (From Atlas of human anatomy, ed 6, Plates 104 and 105.)



Dural Venous Sinuses


The dura also separates to form several large endothelial-lined venous channels between its periosteal and meningeal layers—superior and inferior sagittal, straight, confluence of sinuses, transverse, sigmoid, and cavernous sinuses—and several smaller dural sinuses (Table 8-2 and Fig. 8-7). These dural venous sinuses drain blood from the brain, largely posteriorly, then into the internal jugular veins. These sinuses lack valves, however, so the direction of blood flow through the sinuses is pressure dependent. Of particular importance is the cavernous venous sinus, which lies on either side of the sella turcica and has an anatomical relationship with the internal carotid artery and several cranial nerves, including III, IV, V1, V2, and VI. Injury or inflammation in this region can affect all these important structures. Also, the optic chiasm lies just above this area, so CN II may be involved in any superior expansion of the cavernous sinus (e.g., pituitary tumor).




Subarachnoid Space


The subarachnoid space (between the arachnoid and pia mater) contains cerebrospinal fluid (CSF), which performs the following functions (Figs. 8-5 and 8-8):


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FIGURE 8-8 Relationship of Arachnoid Granulations and Venous Sinus. (From Atlas of human anatomy, ed 6, Plates 101 and 103.)


The arachnoid granulations absorb most of the CSF and deliver it to the dural venous sinuses (see Figs. 8-5 and 8-8). These granulations are composed of convoluted aggregations of arachnoid that extend as “tufts” into the superior sagittal sinus and function as one-way valves for the clearance of CSF; the CSF crosses into the venous sinus, but venous blood cannot enter the subarachnoid space. Small, microscopic arachnoid cell herniations also occur along the spinal cord, where CSF (which circulates at a higher pressure than venous blood) is delivered directly into small spinal cord veins.



Clinical Focus 8-4   Hydrocephalus


Hydrocephalus is the accumulation of excess CSF within the brain’s ventricular system. It is caused by overproduction or decreased absorption of CSF or by blockage of one of the passageways for CSF flow in the subarachnoid space.


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Clinical Focus 8-5   Meningitis


Meningitis is a serious condition defined as an inflammation of the arachnoid and pia mater. It results most often from bacterial or aseptic causes. Aseptic causes include viral infections, drug reactions, and systemic diseases. Patients with meningitis usually present with the following symptoms:



Diagnosis is made by performing a lumbar puncture and examining the CSF.


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Gross Anatomy of the Brain


The most notable feature of the human brain is its large cerebral hemispheres (Fig. 8-9). Several circumscribed regions of the cerebral cortex are associated with specific functions, and key surface landmarks of the typical human cerebrum are used to divide the brain into lobes: four or five, depending on classification, with the fifth either the insula or the limbic lobe. The lobes and their functions are as follows:


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FIGURE 8-9 Brain and Brainstem. (From Atlas of human anatomy, ed 6, Plate 107.)


• Frontal: mediates precise voluntary motor control, learned motor skills, planned movement, eye movement, expressive speech, personality, working memory, complex problem solving, emotions, judgment, socialization, olfaction, and drive.


• Parietal: affects sensory input, spatial discrimination, sensory representation and integration, taste, and receptive speech.


• Occipital: affects visual input and processing.


• Temporal: mediates auditory input and auditory memory integration, spoken language (dominant side), and body language (nondominant side).


• Insula: a fifth deep lobe that lies medial to the temporal lobe (sometimes included as part of temporal lobe); influences vestibular function, some language, perception of visceral sensations (e.g., upset stomach), emotions, and limbic functions.


• Limbic: also sometimes considered a fifth medial lobe (cingulate cortex); influences emotions and some autonomic functions.


Other key areas of the brain include the following components (Fig. 8-9):



Internally, the brain contains four ventricles, two lateral ventricles, and a central third and fourth ventricle (Fig. 8-10). Cerebrospinal fluid, produced by the choroid plexus (see Fig. 8-5), circulates through these ventricles and then enters the subarachnoid space through two lateral apertures (foramina of Luschka) or a median aperture (foramen of Magendie) in the fourth ventricle.


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FIGURE 8-10 Ventricular System of the Brain. (From Atlas of human anatomy, ed 6, Plate 109.)


Blood Supply to the Brain


Arteries supplying the brain arise largely from the following two pairs of arteries (Fig. 8-11 and Table 8-3):



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FIGURE 8-11 Arterial Circle on Base of Brain. (From Atlas of human anatomy, ed 6, Plate 140.)


The vertebral arteries give rise to the anterior and posterior spinal arteries (a portion of the supply to the spinal cord) and the posterior inferior cerebellar arteries, and then join at about the level of the junction between the medulla and pons to form the basilar artery (Fig. 8-11). The internal carotid arteries each give rise to an ophthalmic artery, a posterior communicating artery, a middle cerebral artery, and an anterior cerebral artery. Table 8-3 summarizes the brain regions supplied by these vessels and their major branches.



Clinical Focus 8-6   Subarachnoid Hemorrhage


Subarachnoid hemorrhage usually occurs from an arterial source and results in the collection of blood between the arachnoid and pia mater. The most common cause of subarachnoid hemorrhage is the rupture of a saccular, or berry, aneurysm.


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Clinical Focus 8-7   Epidural Hematomas


Epidural hematomas result most often from motor vehicle crashes, falls, and sports injuries. The blood collects between the periosteal dura and bony cranium. The source of the bleeding is usually arterial (85%); common locations include the frontal, temporal (middle meningeal artery is very susceptible, especially where it lies deep to the pterion), and occipital regions.


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Clinical Focus 8-8   Subdural Hematomas


Subdural hematomas are usually caused by an acute venous hemorrhage of the cortical bridging veins draining cortical blood into the superior sagittal sinus. Half are associated with skull fractures. In a subdural hematoma the blood collects between the meningeal dura and the arachnoid mater. Clinical signs include a decreasing level of consciousness, ipsilateral pupillary dilation, headache, and contralateral hemiparesis. These hematomas may develop within 1 week after injury but often present with clinical signs within hours. Chronic subdural hematomas are most common in elderly persons and alcoholic patients who have some brain atrophy, which increases the space traversed by the bridging veins and renders the stretched vein susceptible to tearing.


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Clinical Focus 8-9   Transient Ischemic Attack


A transient ischemic attack is a temporary interruption of focal brain circulation that results in a neurologic deficit that lasts less than 24 hours, usually 15 minutes to 1 hour. The most common cause of TIA is embolic disease from the heart, carotid, or cerebral vessels, which may temporarily block a vessel. The onset of the deficit is abrupt, and recovery is gradual. The most common deficits include the following:



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Clinical Focus 8-10   Stroke


Cerebrovascular accident (CVA) or stroke is a localized brain injury caused by a vascular episode that lasts more than 24 hours, whereas a transient ischemic attack (TIA) is a focal ischemic episode lasting less than 24 hours. Stroke is classified into the following two types:



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Clinical Focus 8-11   Carotid–Cavernous Sinus Fistula


More common than symptomatic intracavernous sinus aneurysms but less common than subarachnoid saccular (berry) aneurysms, carotid–cavernous sinus fistulas often result from trauma and are more common in men. These high-pressure (arterial) low-flow lesions are characterized by an orbital bruit, exophthalmos, chemosis, and extra-ocular muscle palsy involving CN III, IV, and VI. Blood collecting in the cavernous sinus drains by several venous pathways because the sinus has connections with other dural venous sinuses as well as with the ophthalmic veins and pterygoid plexus of veins in the infratemporal region.


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Clinical Focus 8-12   Collateral Circulation after Internal Carotid Artery Occlusion


If a major artery such as the internal carotid becomes occluded, extracranial and intracranial (circle of Willis) anastomoses may provide collateral routes of circulation. These routes are more likely to develop when occlusion is gradual, as in atherosclerosis, rather than acute, as in embolic obstruction.

Related posts:

  1. Thorax
  2. Abdomen
  3. Upper Limb
  4. Lower Limb

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Jun 16, 2016 | Posted by in ANATOMY | Comments Off on Head and Neck

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