1.1 Step 8: Exposure of Spinal Cord and Roots of Accessory Nerve

These videos have been excerpted from Netter’s Video Dissection Modules on Student Consult. http://bit.ly/2oEt9CO Step 8 Incise the dura mater from the posterior rim of the foramen magnum, and continue the cuts inferiorly on either side. Reflect the dura. Incise the arachnoid membrane to reveal the spinal cord. Remove the occipital bone from the edge of the foramen magnum, inferior to the inferior nuchal line, to fully expose the spinal cord. Having made the initial shallow cuts with the Stryker saw, use a hammer and chisel to separate the bone from the underlying dura and brain stem. With the bone and dura removed, identify the cerebellum, the beginning of the spinal cord, and the eleventh cranial nerve. The eleventh cranial nerve ascends between the dorsal and ventral roots of the spinal nerves. Also note that it lies on the posterior surfaces of the denticulate ligaments. At this point, transect the spinal cord and the accessory nerve at the C1-C2 level to assist in removal of the brain at a later time. Key Terms • Dura mater: a tube of tough connective tissue that surrounds the brain, spinal cord and spinal nerve roots N165 N166. The dura of the spinal canal is continuous with that of the cranial cavity at the foramen magnum N104 N110, but there is no epidural space in the cranial cavity. The spinal dura extends from the foramen magnum to approximately the level of the second sacral vertebra. It is the outer of three layers called meninges. Below the second sacral vertebra, the dura is represented by a tough, fibrous thread, the external filum terminale, which blends with the posterior longitudinal ligament over the coccyx. Thus the dura is anchored above and below. Dural sleeves extend laterally over each of the spinal nerves and blend with connective tissue at the intervertebral foramina N165 N166. • Foramen magnum: the large opening in the posterior cranial fossa N12 N63. Structures passing through it are the spinal cord, accessory nerves, subarachnoid space N110 N128 and the vertebral arteries N137. • Arachnoid mater (arachnoid): the middle of the three meningeal layers N165. This is a thin, filmy layer that forms the inner boundary of the potential subdural space. Its inner surface gives rise to numerous trabeculae that cross the subarachnoid space and become continuous with the pia mater. This gives the appearance of a spider web - hence the name, arachnoid (spider-like). • Occipital bone: a saucer-shaped flat bone situated at the back and lower part of the cranium. It is trapezoidal in shape and curved and is pierced by a large oval aperture, the foramen magnum, through which the cranial cavity communicates with the vertebral canal. N6 N8 N9 • Inferior nuchal line: marks the attachment line of suboccipital muscles (rectus capitis posterior major and minor and the obliquus capitis superior) N172. • Cerebellum: the “little brain” that lies posterior to the pons and occupies most of the posterior cranial fossa. It is joined to the brain