PIA MATER 647 



The chief cLsternip are: (1) the cisterna magna, which is at tlie angle formed Iietwecn the 

 posterior face of the cerebellum and the upper surface of the medulla ol)longata. It communi- 

 cates with the fourth ventricle through the foramen of Magendie (in man); and behind with the 

 wide subarachnoid spac(> of the spinal cord; (2) the cisterna pontis on the ventral surface of the 

 pons; (3) the cisterna loasalis, which lies at the base of the cerel)rum and is divided by the oj)tic 

 chiasma into two parts (cisterna chiasmatis, cisterna interpeduncularis); (4) the cisterna fossie 

 lateralis, situated at the lower part of the lateral fissure (of Sylvius), and continuous with the 

 cisterna basalis. 



Along the upper bortler of the falx cerebri the arachnoidea l)ears bulbous 

 excrescences, the arachnoid granulations or Pacchionian bodies. These are in- 

 closed in thin evaginations of the dura mater and project into the sagittal sinus 

 or the parasinoidal sinuses along either side of it. In some cases they are 

 sufficiently large to exert pressure on the bone and produce in it (le]:)ressions of 

 variable depth. 



The spinal arachnoidea (Arachnoidea spinalis) is directly continuous with that 

 of the l)rain at the foramen magnum. It forms a relatively wide tube around the 

 spinal cord, so that the latter (inclosed in the pia) is surrounded by a very consider- 

 able quantity of cerebrospinal fluid. The spinal subarachnoid space is traversed 

 by fewer arachnoid trabecular than is the case in the cranium. It is partially sub- 

 divided by three imperfect septa. One of these, the septum dorsale, is median and 

 dorsal. The other two, the ligamenta denticulata, are lateral and w'ill he described 

 with the pia mater. 



PIA MATER 



The pia mater is a delicate and very vascular membrane which invests closely 

 the surface of the brain and spinal cord and sends processes into their substance. 

 It also furnishes sheaths to the nerves, w^hich blend outside of the dural sac with the 

 epincurium. 



The pia mater of the brain (Pia mater encephali) follows accurately all the 

 inequalities of the surface, dipping into all the fissures and sulci of the cerebrum, 

 and into the larger fissures of the cerebellum. Its external surface, which forms the 

 inner boundary of the sul)arachnoid space, is covered by a layer of endothelium. 

 From its deep face numerous trabecule are given off which penetrate into the sub- 

 stance of the brain and cord, forming a path for the blood-vessels, and concurring 

 with the neuroglia in forming the supporting tissue of the nervous substance proper. 

 The larger blood-vessels of the brain lie within the subarachnoid space, but the 

 smaller vessels ramify in the pia, forming rich plextises. The twigs which penetrate 

 into the gray matter are inclosed in pial sheaths. They are end-arteries, i. e., 

 constitute the entire supply of the district which they enter and do not anastomose 

 with adjacent vessels. 



Two important folds of the pia extend into the two great transverse fissures 

 of the brain; these processes overlie the third and fourth ventricles and are known 

 as the telae chorioideae of these cavities. They constitute paths for the deeper 

 vessels and their edges contain vascular convolutions which are known as chorioid 

 plexuses. They will receive further consideration more appropriately later. 



The spinal pia mater (Pia mater spinalis) is thicker and denser than that of 

 the brain. It has a strong outer layer of fil)rous tissue, most of the fibers of which 

 are longitudinal. The inner layer is vascular and adheres closel}' to the surface 

 of the cord because it sends numerous processes into it. It sends a fold into the 

 median ventral fissure and also helps to form the median dorsal septum of the cord. 

 Along the median ventral line it forms a band-like thickening, the linea splendens, 

 along which the ventral spinal arter>' nais. On each side the pia mater gives off 

 Si strong longitudinal band, the ligamentum denticulatum, which is connected 

 externally with the dura mater. The inner or pial border extends in a line between 

 the dorsal and ventral roots of the nerves. The outer or dural border is denticu- 



