THE SPINAL ARACHNOID 



919 



transverse fissures of either border, and is continuous anteriorly with the cisterna basaUs and 

 posteriorlv with the subarachnoid cavity about the medulla. 



(3) The cisterna superior hes in the angle between the splenium of the corpus callosum 

 and the superior surfaces of the cerebellum and the mesencephalon, and is connected ventrally, 

 around the cerebral peduncles, with the cisterna basahs. 



(4) The cisterna cerebello-medullaris (cisterna magna) is the cavity between the inferior 

 surface of the cerebellum and the dorsal surface of the medulla oblongata. It is continuous 

 below into the spinal subarachnoid space. The fluid in this cavity is directly continuous with 

 that in the fourth ventricle by way of the foramen of Magendie (median aperture of the fourth 

 ventricle). 



Pacchionian bodies [granulationes arachnoideales] (fig. 724.) — In certain situ- 

 ations, more particularly along the margins of the longitudinal fissure, particu- 

 larly in the frontal region, and to a much less extent upon the superior surface of 

 the vermis of the cerebellum, the subarachnoid tissue elaborates numerous small, 

 ovoid or spherical nodules, the Pacchionian bodies. Each body or arachnoid 

 villus consists of a retiform network of subarachnoid substance and its meshes are 

 filled with cerebro-spinal fluid. The Pacchionian bodies on the vertex of the brain 

 project through the inner layer of the dura mater, both into the superior sagittal 

 sinus and into the venous spaces or parasinoidal sinuses which lie at the sides of 

 that sinus, and, as they become larger, they press against the outer layer of the 

 dura mater and produce ovoid depressions in the inner plate of the cranium. 



Fig. 724.— Coronal Section transverse to the Great Longitudinal Fissure, Showing 

 THE Meninges. (Key and Retzius.) 

 Subarachnoid space Superior sagittal sinus PaccWonian body 



-Corpus callosum 



They probably faciUtate the passage of lymph from the subarachnoid cavity into the 

 blood sinuses, and thus may aid in relieving pressure within. On the other hand, through them 

 the cerebro-spinal fluid is replenished at need from the blood plasma. They are not present 

 at birth, but they appear at the tenth year and increase in number and size with advancing age. 

 They are less marked in the female than in the male. 



The spinal arachnoid (figs. 725, 726) is a loose, reticular sac which is most 

 capacious about the lumbar enlargement of the spinal cord and about the cauda 

 equina. Like that of the encephalon, the portion next to the dura mater alone 

 resembles a membrane, being a loosely organized feltwork, covered on the side of 

 the subdural cavity by a layer of endothelium common to that cavity. Through- 

 out its length the spinal subarachnoid cavity is relatively wide, and, as in the 

 cranium, contains a fine, spong}', web-like reticulum, numerous threads of which 

 are continuous with the pia mater. This spongy tissue is the inner modification 

 of the arachnoid, and its meshes are occupied by the cerebro-spinal fluid. It is 

 not so abundant as in the cranial subarachnoid cavity. 



In addition to the dehcate threads, the arachnoid is more firmly attached to the pia mater 

 by three incomplete partitions. The most continuous of these is arranged along the dorsal 

 mid-line and is known as the septum posticum of Schwalbe (subarachnoid septum). This may 

 be described as a linear accumulation of the spongy tissue which pervades the subarachnoid 

 space. It is most incomplete in the upper cervical region, where it becomes merely a line of 

 threads connecting with the pia. It is most complete as a septum in the lower cervical and in 

 the thoracic region, but at best it maintains a spongy character. The other two partitions are 



