THE PIA MATER OF THE BRAIN Structure and Relations.—It is a vascular membrane com posed of a close network of veins and arteries held together by fibro-elastic areolar tissue (Fig. 9). The endothelium covering its outer surface is continuous with that ensheathing the sub arachnoid trabecula. The pia closely follows the brain surface (Fig. 6). Internally, it sends supporting trabeculm into the brain, which transmit blood-vessels; and externally it forms an investing sheath for each cerebral nerve.
The pia mater and the arachnoid constitute the leptomeninges.
Folds.—Two important processes are formed by the pia mater : (r) The chorioid tela of the third ventricle (tela chorioidea ventriculi tertii) is pushed forward into the anterior part of the transverse fissure of the cerebrum between the fornix and the interbrain (Fig. 6). Hence the old name, velum interpositum. It is triangular in shape, with apex directed forward (Fig. 7). Each lateral border is tucked into the chorioidal fissure of the cerebral hemisphere and enters into the floor of the lateral ventricle, while the median part of the fold is in the roof of the third ventricle. Between the two layers of this chorioid tela is some areolar tissue through which run backward the two in ternal cerebral veins and unite near the base of the tela to form the great cerebral vein. The chorioid plexuses of the lateral and the third ventricles occupy, respectively, the lateral borders and the median area of this chorioid tela. (2) A second fold of pia mater is tucked into the transverse fissure of the cerebellum, dorsal to the medulla oblongata and ventral to the posterior median part of the cerebellum (Fig. 6). It is called the chorioid tela of the fourth ventricle (tela chorioidea ventriculi quarti) because its inferior layer enters into the roof and contains the chorioid plexus of that ventricle. This lower layer invests the posterior surface of the medulla and the roof-epithelium of the fourth ventricle (Fig. 8). It is pierced by three foramina which are situated as follows: One over each lateral angle of the fourth ventricle, the lateral apertures (Key and Retzii, or Lushkm), and one over its inferior angle. The latter is the largest and is called the median aperture (Magendii). Those three foramina establish communication between the posterior subarachnoid space and the fourth ventricle.
The epithelial cells of the chorioid plexuses secrete the cere brospinal fluid and pour it into the ventricles, whence it flows through various apertures into the subarachnoid spaces. In
sufficient flow through the apertures or through the noid outlets results in internal or external hydrocephalus. The cerebrospinal fluid which fills the ventricles, the subarachnoid, subdural and other serous spaces of the central nervous system is not a mere exudate of serum. "It is more like tears and sweat than lymph. It lacks the corpuscle content of lymph; it has only half the alkalinity of lymph; it has no fibrinogen at all and only a mere trace of any protein; and it has from 3—IT per cent. more CO2 than is contained in lymph. Cerebrospinal fluid contains small amounts of sodium chloride, of carbonates, phosphates, urea, 53-61 per cent. of a trace of globulin, of glucose, and, when drawn by lumbar puncture, a few lympho cytes." "Its specific gravity is roo6—roo8." The normal amount present at one time is stated by Mott to be roo-130 cc. It exists under a normal tension of roo-150 mm. of sodium carbonate solution, in the horizontal posture; and of about 400 mm. in the erect posture; this may be increased to 700 mm. or more in pathologic states (Kronig)." (Santee, H. E., 1915: Important Anatomic and Physiologic Factors in Sub arachnoid Medication, Ill. Med. Jour., March.) The arteries of the pia mater supply the brain (Figs. 9, Jo, II and 12). They are the anterior, middle and posterior cere brals; the anterior and posterior chorioidals; and the anterior and posterior inferior cerebellar and the superior cerebellar with many branches.
The veins are more numerous than the arteries in the pia: the internal and great cerebral veins, the veins of the chorioid plexuses of the lateral, third and fourth ventricles and the basilar vein; the cerebral veins; superior, medial and inferior; and the superior and inferior cerebellar veins. All of them empty into the sinuses (see p. 20).
Seven cerebral nerves-3d, 5th, 6th, 7th, 9th, loth and rith—and the sympathetic supply the pia mater and its blood vessels.
The pia mater of the spinal cord has two layers, the outer of which is the more vascular and contains the spinal arteries and the tributaries of the external spinal veins. It forms three processes, namely, the anterior septum, which occupies the anterior median fissure, and the ligamentum denticulatum of each side.