Lab Instructions:

In order to observe the spinal cord and its surrounding structures, we will need to access the vertebral canal (recall that the vertebral canal is the combined space created by the vertebral foramina when the vertebrae are in articulation). To do so, we are going to access the vertebral canal from the dorsal side by removing the vertebral arches (including the spinous processes) of a couple of vertebrae.

In order to observe some regional differences in the spinal cord, we are going to have your teams dissect slightly different vertebrae.

Between the dura of the spinal cord and the periosteum of the vertebrae is the epidural space, which contains loose connective tissue, fat, and blood vessels.

The spinal cord is divided into segments.  A group of dorsal and ventral rootlets leave each spinal cord segment on each side and combine, respectively, to form the spinal nerve at the level of the intervertebral foramen. The dorsal rootlets converge into a spinal ganglion. Note the spinal ganglia in the intervertebral foramina.

There are 8 cervical spinal cord segments, 13 thoracic, 7 lumbar, 3 sacral, and about 5 caudal. The first cervical spinal nerves leave the vertebral canal through the lateral vertebral foramen in the arch of the atlas. The second cervical spinal nerves leave caudal to the atlas. The cervical spinal nerves of segments 3 through 7 leave the vertebral canal through the intervertebral foramina cranial to the vertebra of the same number. The spinal nerves of the eighth cervical segment pass caudal to the seventh (last) cervical vertebra. The spinal nerves of all the remaining spinal cord segments pass through the intervertebral foramina caudal to the vertebra of the same number.

The only spinal cord segments that are found entirely within their corresponding vertebrae are the last two thoracic and the first two (or occasionally three) lumbar segments. All other spinal cord segments reside in the vertebral canal cranial to the vertebra of the same number. This is most pronounced in the caudal lumbar and sacrocaudal segments of the spinal cord. In general, the three sacral segments lie within the fifth lumbar vertebra and the caudal segments lie within the sixth lumbar vertebra. There is breed variation in the length of the spinal cord. In small breeds it extends about one vertebra farther caudally, and in large breeds one vertebra farther cranially. The nerve roots of the first 10 thoracic segments and those caudal to the third lumbar segment are long because of the distance between their origin at the spinal cord and their passage through the intervertebral foramen.

In the caudal cervical region over the fifth to seventh cervical vertebrae, there is an enlargement of the spinal cord that nearly fills the vertebral canal. This is the cervical intumescence. Its presence is due to an increase in white matter and cell bodies that are associated with the innervation of the thoracic limb. This intumescence occurs from the sixth cervical segment of the spinal cord through the first thoracic segment. Another enlargement occurs in the midlumbar vertebral region for the innervation of the pelvic limb. The lumbar intumescence that begins at about the fourth lumbar segment and gradually narrows caudally as the spinal cord comes to an end near the intervertebral space between the sixth and seventh lumbar vertebrae. The narrow caudal end of the parenchyma of the spinal cord is known as the conus medullaris. The spinal cord terminates in the filum terminale, which is a narrow cord of meninges that may include a long extension of the neural tube and central canal. This attaches the conus medullaris to the caudal vertebrae. The cauda equina ([L], “horse’s tail”) includes the conus medullaris together with the adjacent caudal lumbar, sacral, and caudal roots that extend caudally in the vertebral canal. It resembles a horse’s tail, hence “cauda equina”.

Meninges

Returning to your cadaver, observe the thick, fibrous dura mater, the outermost layer of the meninges.

The thin arachnoid membrane and pia mater are opposed to each other in the embalmed specimen, with the arachnoid membrane adhering to the deep surface of the dura mater and the pia mater remaining on the surface of the spinal cord itself. For this reason, upon incising and opening the dura mater, we have now entered the subarachnoid space. Recall that through the subarachnoid space flows cerebrospinal fluid (CSF). On the lateral surface of the spinal cord, the pia thickens and forms a longitudinal cord of connective tissue called the denticulate ligament. This ligament segmentally attaches to the arachnoid and dura laterally, midway between the roots of adjacent spinal cord segments.

For the following material, refer to the calf cadavers made available to you in the lab.

At the level of the atlanto-occipital membrane in the calf, reflecting the muscles of the poll region exposes a piece of tissue known as the dorsal atlanto-occipital membrane, which extends from the occipital bone to the dorsal arch of the atlas (C1). Retracting this membrane exposes the epidural space and dura mater.

Transverse Sections of the Spinal Cord

The gray matter of the spinal cord in transverse section is in the shape of a butterfly or the letter H. It consists primarily of neuronal cell bodies. The dorsal extremity on each side is the dorsal horn, which receives the entering dorsal (sensory) rootlets. The ventral extremity is the ventral horn, which sends axons out via the ventral (motor) rootlets. In the thoracolumbar region, the lateral horn projects laterally from the gray matter midway between the dorsal and ventral gray horns. This lateral horn contains the cell bodies of the preganglionic sympathetic neurons. In the center of the gray matter of the spinal cord is the central canal. This remnant of the embryonic neural tube is continuous rostrally with the fourth ventricle. At the caudal end of the conus medularis, there is a small communication with the subarachnoid space.

The white matter of the spinal cord can be divided into three pairs of funiculi. Dorsally, a shallow, longitudinal groove extends the entire length of the spinal cord. This is the dorsal median sulcus. The longitudinal furrow along which the dorsal rootlets enter the spinal cord is the dorsolateral sulcus. Between these two sulci is the dorsal funiculus of the spinal cord.

Between the dorsolateral sulcus and the line of exit of the ventral rootlets, the ventrolateral sulcus, is located the lateral funiculus. The ventral funiculus is the white matter between the line of exit of the ventral rootlets and the longitudinal groove on the ventral side of the spinal cord, the ventral median fissure. In some species the funiculi have been subdivided topographically into specific cranially and caudally projecting tracts. Such anatomical information in domestic animals is still incomplete.