It must be further remarked that even at those periods of life when the Pacchionian bodies are found in greatest numbers, cases fre quently occur in which no trace of them can be found. There is likewise the greatest variety as to their number and size, in different indivi duals of the same age.
It has always occurred to me to find them most numerous in cases where I had reason to know that the brain had been subject to fre quent excitement during life. In persons ad dicted to the excessive use of spirituous liquors, in those of irritable temperament and who were frequently a prey to violent and exciting passions, they are almost uniformly highly developed.
The Pacchionian bodtes are peculiar to the human subject. Nothing similar to them has been found in any of the inferior classes of animals.
In reference, then, to the question, what is the nature of these bodies, I have no difficulty in stating my opinion that the evidence greatly preponderates in favour of their morbid origin; that they are the product of a chronic very gradual irritation due to more or less frequent functional excitement of the brain itself. It is not unlikely that the friction to which the opposed surfaces of the amchnoid are conti nually subjected in the movements of the brain, especially when they are of a more rapid and violent kind, as under states of cerebral excitement, may contribute to the develope merit of many of the appearances connected with these bodies. The opaque spots which are of such frequent occurrence upon the sur face of the heart may be quoted as an example of a morbid chang,e, very commonly met with, and resulting probably from the friction against each other of opposed serous surfaces. Were the Paechionian bodies normal structures, they would not be so frequently absent from brains which afforded every other indication of being in a healthy state ; nor should we find opacity of the amchnoid (a decidedly unhealthy con dition) so commonly coexistent with the full developemem of them. Again, were they a necessary part of the healthy organism, we might expect to find them more constant as regards size, number, and the extent of surface over which they were placed.
Of the liginnentunt dentatum ( serrated membrane of Gordon).-11iis structure forms a part of the mechanical arrangements con nected with the spinal cord and the roots of its nerves. It is found in the subamclinoid space, adhering on the one hand to the pia mater, and, on the other, attached at certain intervals to the dura mater.
The ligamentum dentatum consists of a nar row longitudinal band, adhering by its inner straight border to the pia mater on each lateral surface of the spinal cord, midway between the anterior and posterior roots of the spinal nerves, reaching from the higliest point in the cervical region down to the filiform prolongation with which it becomes incorporated. Its outer border exhibits a series of tooth-like trian gular processes which are inserted by their apices into the dam mater. The first pointed process, which is longer than the rest and less triangular in shape, is inserted into the dura mater on the margin of the occipital bone, where it stands in relation with some parts of interest. The posterior root of the sub-occi pital nerve, and the filaments of origin and the resultant trunk of the spinal accessory, are on a plane posterior to it. The vertebral artery and the ninth pair of nerves are anterior to it. The number of teeth varies from eighteen to twenty two. The last is attached to the dura mater about the level of the first or second lumbar vertebra. The points of attachment are be tween the points of exit of the spinal nerves, being almost always nearer the lower than the upper nerve. The intervals between each pair of dentated processes vary in different regions of the spine as the distances between the roots of the nerves vary. At its insertion into the dura mater each process pins down the visceml and parietal layers of the arachnoid membrane, probably piercing them to reach the fibrous membrane. At its lowest part, a little above the extremity of the cord, the denticulated margin ceases, and the longitudinal portion may be traced downwards, gradually dimi nishing in size, along each side of the filiform prolongation of the pia mater.