PIA MATER 647 



The chief r-isterna' are: (1) the cisterna magna, which is at the angle formed lietween the 

 posterior face of the cerebellum and tiie upper surface of the medulla oljlongata. It communi- 

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

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

 pons; (3) the cisterna basalis, which lies at the base of the cerebrum and is divided by the optic 

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

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

 cisterna basalis. 



Along the upper border of the falx cerebri the arachnoidea bears bulbous 

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

 closed in thin evaginations of the dura mater and i)roject 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 depressions of 

 variable depth. 



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

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

 spinal cord, so that the latt(T (inclosed in the pia) is stirrounded l)y a very consider- 

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

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

 divided by three imperfect septa. ( )ne of these, the sej)tum dorsale, is median and 

 dorsal. The other two, the ligamenta denticulata, are lateral and will be described 

 with the pia mater. 



PIA MATER 



The pia mater is a delicate and very vascular meml^rane which invests closely 

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

 It also furnishes sheaths to the nerves, which blend outside of the dural sac with the 

 epineuriuni. 



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

 inequalities of the siu'face, dipjiing 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)aracimoid space, is covered by a laj'er of endothelium. 

 From its deep face numerous traljecula? are given oft" 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 su])arachnoid space, but the 

 smaller vessels ramify in the pia, forming rich plexuses. 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 fibrous tissue, most of the fibers of which 

 are longitudinal. The inner layer is vascular and adheres closely 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 artery runs. On each side the pia mater gives off 

 a strong longitudinal band, the ligamenttim 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- 



