The spinal arachnoid, especially in its visceral layer, occupies a different position from that of the cerebral arachnoid, both physiologically and pathologically; for it forms a sac, which does not, as within the skull, adhere closely to the pia mater, but envelopes the spinal cord without being fixed to it. Its position corresponds to that of the cerebral arachnoid, where the latter is stretched across, like a bridge, from one part of the brain to the other. The sac of the spinal arachnoid, therefore, is not single and external only as in the brain, there being also a second space, internal, and enclosed by the visceral layer of the membrane. Generally, however, by the spinal arachnoid sac the inner space is understood. And it is also that which, for its pathological importance, claims, beyond comparison, the most attention; the visceral layer, and the sac within it, being the seat of the most serious diseases of both the inner membranes of the cord, as well as of the products which those diseases furnish.

1. Anomalies In The Extent Or Dimensions Of The Membrane

Besides the uniform enlargements to which the (inner) arachnoid sac is subject from exudations and dropsical effusion into it at various periods of both intra- and extra-uterine life, further anomalies are met with, belonging to the same class; - congenital partial dilatations, which, when combined with local (partial) fission of the vertebral column, represent the disease named Spina bifida. The arachnoid protrudes through an opening in the arches of the vertebrae, - generally an opening left in the mesial line by the deficiency of some lumbar or sacral spinous processes, - and forms a sac or diverticulum, which is filled with serum, and the cavity of which communicates through the neck of the sac with the chamber that contains the cerebro-spinal fluid. The dura mater of the cord, extremely attenuated, is spread over it; but more commonly the arachnoid protrudes through a round or elongated opening in the dura mater also, and is then covered immediately by the general integuments. The sac varies in size, from a scarcely perceptible dilatation to the bulk of a duck's or goose's egg, or it may even be larger. The fulness of the sac, and its consequent tension and fluctuation, are much diminished after death, when it is more or less collapsed. Its base is always constricted, even when broad; and it sometimes forms a pedicle.

The sac is composed of arachnoid membrane, and of general integuments, which are more or less attenuated in proportion to its size, and marked with scars, when it has previously burst. The canal by which its cavity communicates with the internal arachnoid space is short, and always narrow when compared with the area of the protruded sac, and it corresponds with the aperture in the posterior wall of the spinal column, and in the dura mater.

The spinal cord itself may be fissured; more or less of it may be wanting; or it may be quite natural. The nerves may present no further anomaly than that of being stretched longitudinally, but they usually separate from the spinal cord, and, gradually becoming thinner, and wasting, they terminate in the arachnoid, at the base of the sac. This is very commonly the case in somewhat large sacs in the lumbar and sacral regions, in which the dilatation of the arachnoid cavity is aided by a further and important agent in the separation of the nerves, viz., the receding of the spinal cord upward in the latter period of foetal life.

The direct consequences of this vary with the circumstances of the case; there may be slight (Paresen), or complete palsies, which are generally combined with tonic cramp and contraction (club-foot, etc.); or there may be insufficient nutrition. When the tumor is compressed, general convulsions come on, and if the pressure be increased, coma ensues. The general integuments are sometimes so stretched that they inflame and slough, and, in this condition, they rend wide open. The latter accident sometimes occurs during birth, and the sudden evacuation of the contents of the sac is frequently rapidly fatal.

When the nerves and spinal cord are not involved in the disease, it is capable of cure, like the dropsical sacs of the cerebral arachnoid, by being gradually emptied, or by ligature. It may be cured spontaneously by a single, or by repeated opening of the sac, and even by spontaneous separation of the sac, and obliteration of its neck within the fissure in the vertebrae.

In a few cases spina bifida continues to exist up to the period of puberty, and even beyond it: it then not unfrequently increases in size.

It is very commonly, if not always combined with hydrocephalus, and thereby with hydrencephalocele, and hemicephalus.

The origin of spina bifida is traceable to the same cause which gives rise to dropsical sacs of the cranial arachnoid. A congenital dropsy of the arachnoid sac sometimes occurs, which is closely allied to spina bifida, and in which, whilst the sac is uniformly dilated, the vertebral canal maintains its integrity.

2. Diseases Of Its Texture

1. Hyperoemia. Apoplexy. Inflammation (Arachnitis Spinalis)

No congestion of the spinal arachnoid occurs, or inflammation of it to any extent, without a similar condition of the pia mater of the cord: and, as the vascularity is much more marked on the latter membrane, the description of these affections and of their products may be postponed till the diseases of the pia mater are taken into consideration.

There is, however, one condition, - a consequence of habitual, long-continued, or repeated congestions, and of slight attacks of inflammation, which, as it is an affection of the arachnoid membrane almost alone, demands notice in this place. It consists of dulness, opacity, and thickening of the arachnoid, and is usually combined with chronic effusions of serum into its sac (mostly the inner sac). These changes are sometimes diffused uniformly over a certain portion of the membrane; at other times they are developed at separate spots, and form insulated patches (plaques): but very slight indications, if any, ever occur of a granular thickening, corresponding to the Pacchionian bodies observed in the cerebral arachnoid. The lumbar portion of the membrane is the most affected; and there, indeed, habitual congestions most frequently occur. The so-called ossification of the spinal arachnoid has some connection with these congestions.

Adhesions between the visceral layer and the dura mater rarely take place, except as the result of local, and especially of traumatic, inflammations of the arachnoid.

2. Adventitious Growths

All forms of growth are very rare in this membrane; except bone, which forms a marked exception to the rule.

It is always found on the visceral layer of the membrane, and forms scales and plates, which are generally of small size; ranging between that of a poppy-seed and that of a lentil, and but rarely equalling a pea or a silver threepenny-piece (Groschen). They are cartilaginous, or cartilaginous and bony, flexible, and of a white, or yellowish-white color. They are bent towards the periphery of the cylinder, and appear convexo-concave; and the larger they are, the more distinct is this peculiar form. They are imbedded in the arachnoid, but they project from the membrane inward rather than outward; their inner surface is rough, the outer smooth. Their number is very various; sometimes at one, or a few scarcely perceptible points only, the commencement of the bony growth can be detected; in other cases they are very numerous. Their usual situation is the lumbar and lower dorsal regions; but, in a few instances, when they are very numerous, they may extend as high as the upper dorsal region.

Ollivier speaks also of cancerous tumors almost free in the arachnoid sac.

3. Anomalous Contents Of The Spinal Arachnoid

As these are principally products of the vascular system of the pia mater, I shall treat of them among the diseases of that membrane. For the same reason, the morbid effusions are almost always confined to the inner arachnoid sac.