Introduction. I. Accumulation of cerebro-spinal fluid. 1. (Edema of membranes. 2. Hydrocephalus, acquired and congenital. 3. Dropsies of central canal and meninges of cord, (a) Hydrorrhachis interna, hydromyelia, syringomyelia, (b) Spina bifida, in various forms; the tumour of spina bifida; condition of the cord; spina bifida occulta. II. Meningeal haemorrhage.
2. Leptomeningitis, including (a) simple, acute, and chronic, (b) epidemic cerebro-spinal, and (c) tubercular meningitis. IV. Tumours of the meninges. V. Affections of the Pineal and Pituitary bodies.
The Dura mater is a dense connective-tissue membrane which has much more intimate connections with the bones than with the brain and spinal cord, forming, in fact, a periosteum to the bone. On its internal surface there is no separate arachnoid membrane, but the dura mater is less vascular here than in its external layers. The space between the dura mater and the surface of the brain is sometimes described as a serous cavity analogous to the pleura or peritoneum, but in its pathological relations, at least, it is very different, and it is better designated as the Subdural space. The internal surface of the dura mater and the external surface of the arachnoid are defective in vessels, and are hence little liable to inflammations. This cavity forms, in fact, a kind of barrier to the propagation of inflammations either from without or within. Hence inflammations of the external surface of the dura mater seldom extend to its internal surface, and it is very rare to meet with an inflammation affecting the whole subdural space. On the other hand, the arachnoid and pia mater really form one membrane, which is partly in two layers. The deep layer or pia is closely connected with the surface of the brain and spinal cord, following the various irregularities of the surface. This deep layer is connected with the superficial one - the arachnoid - more or less intimately. In the sulci between the convolutions there is loose connective tissue with many interstices, constituting the Subarachnoid space. But on the summits of the convolutions the two membranes are intimately united. The free surface of the arachnoid is, like that of the dura mater, defective in vessels, and inflammations of the deeper layer scarcely ever extend to this surface. We have thus to distinguish the dura mater connected with the bone, the subdural space, the pia-arachnoid forming the proper meninges of the brain, and the subarachnoid space.
The Cerebrospinal fluid is contained partly in the ventricles of the brain and central canal of the spinal cord on the one hand, and partly in the Subarachnoid space on the other. The subarachnoid space at the base of the brain and over the spinal cord is specially well-marked. The brain and cord may be said to be submerged in the fluid contained in these spaces, and the brain, more particularly, lies on a kind of " water-bed " formed by the more voluminous spaces at the base filled with the fluid. The amount of cerebro-spinal fluid as a whole is liable to alterations according as the volume of the brain and the fulness of the blood-vessels vary. The dtsti'ibution of the fluid also varies between the different parts of the system, more particularly between the ventricles and the subarachnoid space. Between these two there are important communications. The pia mater is prolonged into the lateral ventricles through the transverse fissure, as the velum interpositum and choroid plexus, thus forming a somewhat free communication. There is also an open communication between the lower part of the fourth ventricle and the subarachnoid space, by the Foramen of Majendle. The subarachnoid space and the ventricles of the brain with the central canal of the spinal cord thus form a continuous system of lymph spaces, and the cerebro-spinal fluid circulates in them. In addition to this, the subarachnoid space communicates with lymph spaces around the vessels and ganglion cells, the Perivascular and Periganglionic spaces. It is important to observe that the subarachnoid space does not communicate with the subdural space.