Chapter 5
Section 5 of Grey's Anatomy Part 4. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recording by M. L. Cohen. Anatomy to Human Body, Part 4 by Henry Grey.
The Spinal Cord, Part 1. The Medulla Spinalis or Spinal Cord. The Medulla Spinalis or Spinal Cord forms the elongated, nearly cylindrical part of the central nervous system which occupies the upper two-thirds of the vertebral canal. Its average length in the male is about 45 cm, in the female from 42 to 43 cm, while
its weight amounts to about 30 g. It extends from the level of the upper border of the atlas to that of the lower border of the first or upper border of the second lumbar vertebra. Above, it is continuous with the brain. Below, it ends in the conical extremity, the conus medullaris, from the apex of which a
delicate filament, the phylum terminale, descends as far as the first segment of the coccyx. The position of the Medulla Spinalis varies with the movements of the vertebral column, its lower extremity being drawn slightly upward when the column is flexed. It also varies at different periods of life. Up to the third month of fetal life, the Medulla Spinalis is as long as the vertebral canal,
but from this stage onward, the vertebral column elongates more rapidly than the Medulla Spinalis, so that by the end of the fifth month, the Medulla Spinalis terminates at the base of the sacrum, and at birth, about the third lumbar vertebra. The Medulla Spinalis does not fill the part of the vertebral canal in which it lies. It is in sheathed by three protective membranes, separated from each other by two concentric
spaces. The three membranes are named from without inward, the dura mater, the arachnoid, and the pia mater. The dura mater is a strong fibrous membrane which forms a wide tubular sheath. This sheath extends below the termination of the Medulla Spinalis, and ends at a point
cul-de-sac at the level of the lower border of the second sacral vertebra. The dura mater is separated from the wall of the vertebral canal by the epidural cavity, which contains a quantity of loose areolar tissue, and a plexus of veins. Between the dura mater and the subjation arachnoid is a capillary interval, the subdural cavity, which contains a small quantity of fluid, probably of the nature of lymph.
The arachnoid is a thin transparent sheath separated from the pia mater by a comparatively wide interval, the subarachnoid cavity, which is filled with cerebrospinal fluid. The pia mater closely invests the Medulla Spinalis, and sends delicate septae into its substance. A narrow band, the ligamentum denticulatum, extends along each of the lateral surfaces, and is attached by a series of pointed processes to the inner surfaces of the dura mater.
Thirty-one pairs of spinal nerves spring from Medulla Spinalis, each nerve having an anterior or ventral, and a posterior or dorsal root, the latter being distinguished by the presence of an oval swelling, the spinal ganglion, which contains numerous nerve cells. Each root consists of several bundles of nerve fibers, and at its attachment extends for some distance along the side of the Medulla Spinalis.
The pairs of spinal nerves are grouped as follows, cervical VIII, thoracic XII, lumbar V, sacral V, coccidial I, and for convenience of description, the Medulla Spinalis is divided into cervical, thoracic, lumbar, and sacral regions, corresponding with the attachments of the different groups of nerves. Although no trace of transverse segmentation is visible on the surface of the Medulla Spinalis,
it is convenient to regard it as being built up in a series of superimposed spinal segments or neuromeres, each of which has a length equivalent to the extent of attachment of a pair of spinal nerves. Since the extent of attachment of the successive pairs of nerves varies in different parts, it follows that the spinal segments are of varying lengths.
Thus, in the cervical region, they average about 13 millimeters, in the mid-thoracic region about 26 millimeters, while in the lumbar and sacral regions they diminish rapidly from about 15 millimeters at the level of the first pair of lumbar nerves to about 4 millimeters opposite the attachment of the lower sacral nerves. As a consequence of the relevant inequality in the rates of growth of the Medulla Spinalis
and vertebral column, the nerve roots, which in the early embryo pass transversely outward to reach their respective intervertebral foramenot, become more and more oblique in direction from above downward, so that the lumbar and sacral nerves descend almost vertically to reach their points of exit. From the appearance, these nerves present at the attachment to the Medulla Spinalis,
and from their great length, they are collectively termed the cauda aquina. The phylum terminale is the delicate filament about 20 centimeters in length, prolonged downward from the apex of the conus medullaris. It consists of two parts, an upper and a lower. The upper part, or phylum terminale internum, measures about 15 centimeters in length and
reaches as far as the lower border of the second sacral vertebra. It is contained within the tubular sheath of dura mater and is surrounded by the nerve forming the cauda aquina, from which it can be readily recognized by its bluish-white color. The lower part, or phylum terminale externum, is closely invested by and is inherited to
the dura mater. It extends downward from the apex of the tubular sheath and is attached to the back of the first segment of the coccyx. The phylum terminale consists mainly of fibrous tissue, continuous above with that of the pia mater.
Adhering to its outer surface, however, are a few strands of nerve fibers which probably represent rudimentary second and third coccygeal nerves. Further, the central canal of the Medulla Spinalis extends downward into it for 5 or 6 centimeters. Enlargements
The Medulla Spinalis is not quite cylindrical, being slightly flattened from before backward. It also presents two swellings or enlargements, an upper or cervical and a lower or lumbar. The cervical enlargement is the more pronounced and corresponds with the attachment of the large nerves which supply the upper limbs. It extends from about the third cervical to the second thoracic vertebra.
Its maximal circumference, about 38 millimeters, being on a level with the attachment of the sixth pair of cervical nerves. The lumbar enlargement gives attachment to the nerves which supply the lower limbs. It commences about the level of the ninth thoracic vertebra and reaches its maximum circumference of about 33 millimeters opposite the last thoracic vertebra, below which it
tapers rapidly into the conus medullaris. Fissures and Sulci An anterior median fissure and a posterior median sulcus incompletely divide the Medulla Spinalis into two symmetrical parts which are joined across the midline by a commissural band of nervous matter.
The anterior median fissure, Fissura Mediana Anterior, has an average depth of about 3 millimeters, but this is increased in the lower part of the Medulla Spinalis. Contains a double fold of pia mater and is florist formed by a transverse band of white substance, the anterior white commissure, which is perforated by blood vessels on their way to or from the central part of the Medulla Spinalis.
The posterior median sulcus, Sulcus Medianus Posterior, is very shallow. From it, a septum of Neuroglia reaches rather more than halfway into the substance of the Medulla Spinalis. This septum varies in depth from 4 to 6 millimeters, but diminishes considerably in the lower part of the Medulla Spinalis.
On either side of the posterior median sulcus and at a short distance from it, the posterior nerve roots are attached along a vertical furrow named the posterior lateral sulcus. The portion of the Medulla Spinalis which lies between this and the posterior median sulcus is named the posterior funiculus. In the cervical and upper thoracic regions, this funiculus presents a longitudinal furrow,
the posterior intermediate sulcus. This marks the position of a septum which extends into the posterior funiculus and subdivides it into two fasciculi, a medial named the fasciculus gracilis, tract of gall, and a lateral, the fasciculus cuneatus, tract of burdach. The portion of the Medulla Spinalis which lies in front of the posterior lateral sulcus
is termed the anterior lateral region. The anterior nerve roots, unlike the posterior, are not attached in linear series, and their position of exit is not marked by a sulcus. They arise by separate bundles which spring from the anterior column of the gray substance and, passing forward through the white substance, emerge over an area of some slight width.
The most lateral of these bundles is generally taken as a dividing line which separates the anterior lateral region into two parts, that is, an anterior funiculus between the anterior median fissure and the most lateral of the anterior nerve roots, and a lateral funiculus between the exit of these roots and the posterior lateral sulcus. In the upper part of the cervical region, a series of nerve roots passes outward through
the lateral funiculus of the Medulla Spinalis. These unite to form the spinal portion of the accessory nerve which runs upwards and enter the cranial cavity through the foranum and magnum. The internal structure of the Medulla Spinalis. On examining a transverse section of the Medulla Spinalis, it is seen to consist of gray and
white nervous substance, the former being enclosed within the latter. Gray substance. Substantia grisiocentralis. The gray substance consists of two symmetrical portions, one in each half of the Medulla Spinalis.
These are joined across the middle line by a transverse commissure of gray substance through which runs a minute canal, the central canal, just visible to the naked eye. In a transverse section, each half of the gray substance is shaped like a comma or crescent, the concavity of which is directed laterally, and these together with the intervening gray commissure present the appearance of the letter H.
An imaginary coronal plane through the central canal serves to divide each crescent into an anterior or ventral and a posterior or dorsal column. The anterior column, columnar anterior, anterior corneu, directed forward is broad and of a rounded or quadrangular shape. Its posterior part is termed the base and its anterior part the head, but these are
not differentiated from each other by any well-defined constriction. It is separated from the surface of the Medulla Spinalis by a layer of white substance which is traversed by the bundles of the anterior nerve roots. In the thoracic region, the posterior lateral part of the anterior column projects lateral words as a triangular field which is named the lateral column, columnar lateralis, the
lateral corneu. The posterior column, columnar posterior, posterior corneu, is long and slender and is directed backwards in lateral words. It reaches almost as far as the posterior lateral sulcus from which it is separated by a thin layer of white substance, the tract of Lissauer.
It consists of a base, directly continuous with the base of the anterior horn, and a neck or slightly constricted portion which is succeeded by an oval or fusiform area termed the head of which the apex approaches the posterior lateral sulcus. The apex is capped by a V-shape or crescentic mass of translucent gelatinous neuroglia termed the substantia gelatinosa of Rolando which contains both neuroglial cells and small nerve
cells. Between the anterior and posterior columns, the gray substance extends as a series of processes into the lateral funiculus to form a network called the formatio reticularis. The quantity of gray substance, as well as the form which it presents on transverse section, varies markedly at different levels.
In the thoracic region, it is small not only in amount, but relatively to the surrounding white substance. In the cervical and luminar enlargements, it is greatly increased. In the latter and especially in the conus medullaris, its proportion to the white substance is greatest.
In the cervical region, its posterior column is comparatively narrow, while its anterior is broad and expanded. In the thoracic region, both columns are attenuated, and the lateral column is evident. In the conus medullaris, the gray substance assumes the form of two oval masses, one in each half of the cord, connected together by a broad gray commissure.
The central canal, canalus centralis, runs throughout the entire length of medulla spinalis. The portion of gray substance in front of the canal is named the anterior gray commissure, that behind it, the posterior gray commissure. The former is thin and is in contact anteriorly with the anterior white commissure. It contains a couple of longitudinal veins, one on either side of the middle line.
The posterior gray commissure reaches from the central canal to the posterior median septum and is thinnest in the thoracic region and thickest in the conus medullaris. The central canal is continued upward through the lower part of the medulla oblongata and opens into the fourth ventricle of the brain. Below, it reaches for a short distance into the phylum terminale.
In the lower part of the conus medullaris, it exhibits a fusiform dilatation, the terminal ventricle. This has a vertical measurement of from 8 to 10 millimeters, is triangular on cross-section, with its base directed forward and tends to undergo obliteration after the age of 40 years.
Throughout the cervical and thoracic regions, the central canal is situated in the anterior third of the medulla spinalis. In the lumbar enlargement, it is near the middle, and in the conus medullaris, it approaches the posterior surface. It is filled with cerebrospinal fluid, inline by ciliated columnar epithelium, outside of
which is an encircling band of gelatinous substance, the substantia gelatinosa centralis. This gelatinous substance consists mainly of neuroglia, but contains a few nerve cells and fibers. It is traversed by processes from the deep ends of the columnar ciliated cells which line the central canal.
End of section 5.
