PIA MATER 763 
but consists of a relatively small number of long, thread-like strands which traverse 
the cavity. 
The chief cistern are: (1) the cisterna 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 bebind 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 fosse lateralis, 
situated at the iower 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 trabecule 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 trabecule are given off which penetrate into the sub- 
stance of the brain, 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 plexuses. The twigs which penetrate into 
the gray matter are enclosed in pial sheaths. They are end-arteries, 7. 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 brain. 
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-like 
1 They are also known as Pacchionian bodies. 
