PIA MATER 763 



but consists of a relatively small number of long, thread-like strands which traverse 

 the cavity. 



The chief cistemae are: (1) the oistema magna, which is at the angle formed between the 

 posterior face of the cerebellum and the dorsal surface of the medulla oblongata. It communi- 

 cates with the fourth ventricle through lateral openings in the latter, and behind with the wide 

 subarachnoid space of the spinal cord; (2) the cistema 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, cistema interpeduncularis) ; (4) the cisterna fossae lateralis, 

 situated at the lower part of the lateral fissure and continuous with the cisterna basalis. 



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

 excrescences, the arachnoid granulations.^ These are enclosed in thin evaginations 

 of the dura mater and project into the dorsal longitudinal 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 continuous with that of the 

 brain at the foramen magnum. It forms a relatively wide tube around the spinal 

 cord, so that the latter (enclosed in the pia) is surrounded by a very considerable 

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

 fewer trabeculse than is the case in the cranium. It is partially subdivided by three 

 imperfect septa. One of these, the septum 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 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, which blend outside of the dural sac with the 

 epineurium. 



The cerebral pia mater (Pia mater encephali) follows accurately all the in- 

 equalities 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 subarachnoid space, is covered by a layer of endothelium. 

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

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

 neurogha 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 plexuses. The twigs which penetrate into 

 the gray matter are enclosed 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. 



Important folds of the pia extend into two of the great fissures of the bram. 

 One of these passes in at the transverse fissure between the cerebellum and 

 the cerebral hemispheres, and is continued so as to overlie the third ventricle; it 

 forms the tela chorioidea of that cavity. Another fold passes in at the fissure 

 between the cerebellum and the medulla oblongata and forms the tela chorioidea 

 of the fourth ventricle. 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 spinal cord, because numerous processes extend into the latter from 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-hke 

 1 They are also known as Pacchionian bodies. 



