Pacchionian Granulations

PACCHIONIAN GRANULATIONS, Pacchioni granulationes (synonyms: corpuscula, glandulae Pacchioni), are pear-shaped or club-shaped growths scattered on the outer surface of the arachnoid membrane of the brain and connected to it by more or less thin stalks or pedicles. These arachnoidal formations are found exclusively in the brain, predominantly on the surface of the hemispheres; a much smaller number of them are found on the cerebellum. They are always located in the region of the sinuses, as well as the parasinusoidal venous spaces (lacunae venosae of Key and Retzius) located near the superior sagittal sinus (see separate table, figure 3). Pacchionian granulations are found in the greatest number in the superior sagittal sinus and its tributaries—the transverse, cavernous, and superior petrosal sinuses, and the middle meningeal veins. If the dura mater is opened above the parasinusoidal space or above the sinus itself, the floor of these venous reservoirs often appears as if paved with granulations. The villi, growing into such venous spaces, press the inner layer of the dura mater before them and thin it out to such an extent that almost nothing remains of it except the endothelium. Between this endothelium and the endothelium covering the villi, clefts remain. Colored fluid injected into the subdural space easily enters the subdural cavities of the Pacchionian granulations through these clefts, from where it also penetrates into the cavity of the venous sinuses. On the other hand, if fluid is injected into the subarachnoid space, it rapidly fills all the granulations, oozes through their endothelial covering into the subdural space of the villi, and further passes into the venous sinuses. Thus, fluid enters the venous sinuses from both the subdural and subarachnoid spaces. These experiments, first performed by Key and Retzius, show with certainty that a connection between the circulation of the cerebrospinal fluid and the venous blood flow is established through the agency of Pacchionian granulations. Regarding the role of granulations in the sense of brain statics in relation to the skull, see Craniocerebral topography. Pacchionian granulations are similar in their histological structure to the arachnoid. They are composed of networks of trabeculae, in the spaces between which there are numerous cavities, giving the entire tissue a cavernous appearance. The trabeculae themselves consist of bundles of loose connective tissue covered with endothelium. The trabeculae located on the periphery of the villus expand into plaques that closely adhere to each other and form a sort of continuous membrane with a small number of clefts leading into the interior of the villus itself. The outer surface of the granulations is covered with endothelium, which constitutes a continuation of the arachnoid endothelium. In children, Pacchionian granulations may be completely absent; up to the age of ten, they are poorly expressed. Sometimes several villi sit together on a single stalk, as a result of which they acquire very diverse shapes. Some of the granulations reach such large sizes that they form rather large depressions, or erosions (impressiones digitatae), in the adjacent cranial bones. However, some authors (Obersteiner) consider not so much the villi themselves as the venous diverticula to be the cause of these erosions.