THE ARACHNOID OF THE BRAIN (Arachnoidea Encaphali) The arachnoid is present only in mammals. It is produced by the delamination of the pia mater, as that is seen in birds and reptiles. In structure it is a delicate, fibrous, web-like net covered externally with endothelium. Internally it is joined to the pia mater by innumerable fibrous trabeculx, the sub arachnoid tissue (Fig. 4) because of the incomplete cleavage of the pia mater. The trabeculx are ensheathed and all sub arachnoid spaces lined with a single layer of endothelial cells, hence both surfaces of the aTchnoid are formed of endothelium. Conical elevations of fibrous tissue with their investing endo thelium constitute the arachnoid villi seen on the outer surface.' Relations.—The arachnoid follows the inner surface of the dura and is prolonged, as a sheath, upon the nerves which pierce it. It does not dip into the sulci of the cerebrum (Fig. 6) ; but only into the transverse, the lateral and the longitudinal fissures, and does not reach to the bottom of the latter. From the pia it is separated by the subarachnoid spaces (cava subar achnoidealia). The anterior subarachnoid space (Fig. 6) includes the cisterna pontis, c. interpeduncularis, c. ambiens, c. chiasma tis, and the c. fosse lateralis cerebri. It is located in front of the medulla, pons and mid-brain and in the lateral fossa and fissure. The posterior subarachnoid space (Fig. 6) is located be hind the medulla and cerebellum. It embraces the cisterna cere bellomedullaris, a space between the medulla and cerebellum, and the cisterna venom cerebri magnet, situated under the splenium of the corpus callosum and along the great cerebral vein. These two, the anterior and posterior, are the largest
subarachnoid spaces and they contain much of the subarach noid fluid. But in the cerebral sulci and fissures there are streams of this fluid which constitute the subarachnoid rivulets. The anterior subarachnoid space has slit-like communications with the inferior horn of the lateral ventricle; the posterior space communicates with the fourth ventricle (Fig. 8) through the median aperture (apertura mediana ventriculi quarti, M a gendii) and the lateral apertures (apertura lateral ventriculi par ti , Key and Retzii or Lushkce).
The hypertrophied villi of the arachnoid, which project into the sinuses, and the perivascular channels of the capillaries and veins of the brain form the outlets for the subarachnoid fluid into the blood stream.
The vessels seen for a short distance in the arachnoid belong to the pia mater. Its nerves are doubtful. Perhaps branches of the mandibular, of the facial and of the accessory supply it.
In the arachnoid of the cord fewer trabeculm join it to the pia; and these, in great part, are collected to form a fenestrated sep tum in the posterior median line (Fig. 97, A). The external spinal veins are covered by the spinal arachnoid, they lie between it and the pia.