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):





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.




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):





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):




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






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):




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):






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):




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.





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:




• 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.




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):





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.








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