The large extent of articular surface in the carpus, is deserving of notice. The different joints should be opened by the student. This can be done after the soft parts have been dissected, without injuring the bones.

SECT. V.-DISSECTION OF THE SPINAL CANAL AND ITS CONTENTS.

To lay open the spinal canal, the soft parts which cover the lamella of the vertebræ should be entirely removed. When this has been done, a mallet and sharp chisel may be used for dividing the lamella on each side of the spinous processes; or, instead of the chisel, a saw may be used for this purpose. Bone-nippers will also sometimes be found useful. The ligamentous attachments must be severed with the scalpel. The vertebral column should be made as convex, posteriorly, as possible, by placing blocks underneath the subject. If the calvaria and the posterior part of the occipital bone have been removed before opening the canal, the student will be able to obtain a very satisfactory view of the relations of the parts in the spinal canal to those in the cranial cavity.

The contents of the spinal canal are the following: The dura mater, the arachnoid, the pia mater, the spinal cord, the roots of the spinal nerves, the spinal ganglia, and the intra-spinal vessels.

The DURA MATER of the cord has the same structure as that of the brain, with which it is continuous through the occipital foramen; it has not, however, the same uses. does not form an internal periosteum to the walls of the spinal canal, nor does it furnish sinuses for the transmission of venous blood, or send off processes to support different parts of the spinal marrow.

It is separated from the parietes of the canal, more or less, by a soft, reddish, adipose and areolar tissue, and by plexuses of veins. It has fibrous attachments to the posterior common spinal ligament. Its external surface is generally smooth. In size it corresponds to the cord, being larger in the neck and the loins than elsewhere. It gives off processes, which surround the nerves as they pass through the intervertebral foramina. The lower part of it is divided into tubular pro

longations, which contain the sacral nerves as they extend some distance in the canal before leaving it. Below it sends down a small fibrous cord, which is attached to the walls of the lower part of the sacral canal. It is more fixed in the anterior than in the posterior part of the canal, by its attachments to the posterior common spinal ligament.

The ARACHNOID lines the internal surface of the dura mater, and also invests the cord. It is exposed by slitting open the dura mater in the median line, through its whole length. It is continuous with the arachnoid of the brain, to which it is similar in structure and function. The visceral portion is connected to the parietal by tubular prolongations around the roots of the nerves.

It is separated from the pia mater by the subarachnoid space, which is occupied by cellulo-fibrous tissue and a fluid. The fibrous structure is most abundant in the middle line behind, where it forms an imperfect septum. If this space be punctured in a living animal, the subarachnoid fluid escapes in a jet, causing, for a time, stupefaction of the animal. The principal use of this fluid is, probably, to protect the spinal marrow. A serous exhalation takes place from both surfaces of the arachnoid membrane, where it invests the cord.

The PIA MATER surrounds and adheres closely to the spinal cord. It is continuous with that of the brain, from which, however, it differs in structure, being more dense and fibrous, and less vascular. It sends prolongations into the anterior and posterior fissures of the cord, and also along the roots of the nerves. It terminates below, after furnishing sheaths for the nerves, in a fibrous cord, which descends to the sacrum, where it is attached to the dura mater.

The LIGAMENTUM DENTICULATUM, Fig. 126 (19), is found on each side of the cord, and between the anterior and posterior roots of the nerves; it reaches from the occipital foramen to the first lumbar vertebra. Its inner border is straight, and appears to be blended with the pia mater; while its outer border presents a series of tooth-like processes, which are attached to the dura mater in the spaces between the foramina which give exit to the nerves. In the upper part of the canal the spinal accessory nerve lies behind it. The serrated processes number twenty one or two. It separates the roots of the nerves and prevents lateral movement of the cord.

The intra-spinal vessels consist of those which supply the walls of the canal, and the cord, including its membranes.

Fig. 126.

The ARTERIES are derived from the vertebral, the intercostal, the lumbar, and the lateral sacral. Those which are denominated the anterior and posterior spinal arteries arise from the vertebral near the occipital foramen. There are two posterior and one anterior. They supply, principally, the upper part of the cord. The remaining arteries enter the canal through the intervertebral foramina, and reach the cord by passing along the roots of the nerves. There are usually three or four branches much larger than the others. These form a free anastomosis with the anterior and posterior spinal arteries.

The VEINS of the cord leave the canal by passing through the occipital and intervertebral foramina. Those which enter the cranial cavity terminate in the sinuses of the dura mater; the others are connected with the sacral, the lumbar, the intercostal, and the vertebral veins. There are several plexuses of veins in the spinal canal outside of the dura mater. These are designated the anterior, the lateral, and the posterior plexuses. There are two plexuses which extend the whole length of the canal. They are situated on the posterior surface of the bodies of the vertebræ, one on each side of the posterior common ligament. These plexuses all communicate freely with each other.

AN ANTERIOR VIEW OF THE SPINAL MARROW, SEEN IN ITS WHOLE LENGTH, AFTER REMOVAL FROM THE SPINAL CANAL.-1. Lines indicating the corpora pyramidalia. 2. Corpora olivaria. 3. Anterior face of the spinal marrow. 4. Anterior roots of the cervical spinal nerves. 5. Anterior roots of the dorsal nerves. 6. Anterior roots of the lumbar nerves. 7. Anterior roots of the sacral nerves. 8, 9, 10, 11. The anterior and posterior roots of the spinal nerves, united to pass out of the dura mater. 12. Dura mater of the medulla spinalis. 13. Ganglia on the cervical nerves. 14. Ganglia on the dorsal nerves. 15. Ganglia on the lumbar nerves. 16. Ganglia on the sacral nerves. 17. Cauda equina. 18. Sub-occipital nerve. 19. Ligamentum denticulatum.

THE MEDULLA SPINALIS, OR SPINAL CORD.

The spinal cord, Fig. 126, extends from the occipitoatlantoid articulation, to the first lumbar vertebra; being from sixteen to eighteen inches in length. It is much smaller than the spinal canal. It is not of equal size throughout its whole length. Those portions which correspond to the roots of the nerves that supply the upper and lower extremities, are the largest. It terminates below in the cauda equina, Fig. 126 (17), which consists of nervous cords.

The cord presents, externally, an anterior and a posterior median fissure, Fig. 128, which

The

divide it, externally, into two
distinct lateral columns.
pia mater enters these fissures.
The anterior one is the widest,
while the posterior is the
deepest. A lateral fissure is
observed where the posterior
roots of the nerves are attach-
ed to the cord. This fissure
divides each half of the cord
into an antero-lateral, and a
posterior column. Another
lateral fissure has been de-
scribed, corresponding to the
anterior roots of the nerves;
this, however, is scarcely per-
ceptible.

When a transverse section of the cord is made, each half is seen to consist, internally, of gray substance, Fig. 128. This is arranged so as to present a semilunar form, the cornua of which correspond to the anterior and posterior roots of the nerves. The posterior cornu reaches the external surface of the cord, but the anterior, which is the shortest and thickest of the two, does not. From this arrangement of the gray substance each half of the cord may be said to consist of three columns, the anterolateral being divided into two by the anterior cornu. Vesicular matter exists in both cornua, but is most abundant in the

posterior. The gray substance in the anterior cornu has been called the substantia gelatinosa; and that in the posterior, the substantia spongiosa.

B

The two halves of the cord are united by a gray commissure at the bottom of the posterior median fissure, and a white commissure at the bottom of the anterior median fissure.

A

C

D

Fig. 128.

E

F

The relative proportions of the gray and white substance vary somewhat in differ

ent sections of the cord.

The precise manner in which the spinal nerves are connected with the substance of the cord, is not well understood. The anterior and posterior roots approach each other as they proceed from the cord to the foramina in the dura mater, Fig. 126 (8, 9, 10, 11), through which they pass separately. The posterior root or fasciculus of each nerve enters a ganglion, Fig. 127 (3), in the intervertebral foramen; and almost immediately after passing through the ganglion it joins the anterior fasciculus, and is inclosed with it in the same neurilemma, which is derived from the dura mater. The fasciculi increase in length from above downwards before they perforate the fibrous sheath of the cord. The posterior roots are sensor, and are larger than the anterior, which are motor, Fig. 127 (1, 2). Each set is supposed to contain filaments, which belong to both the excito-motor and the cerebro-spinal system of nerves.

TRANSVERSE SECTION OF THE SPINAL CORD.-A. Immediately below the decussation of the pyramids. B. At middle of cervical bulb. c. Midway between cervical and lumbar bulbs. D. Lumbar bulb. E. An inch lower. F. Very near the lower end. a. Anterior surface. p. Posterior surface. The points of emergence of the anterior and posterior roots of the nerves are also seen.