MENINGES 439 



veins from the substance of the brain, but the arteries of the dura, or 

 meningeal arteries, supply the cranial periosteum. The dura has many 

 nerves, some with free endings, and others supplying the musculature of 

 the vessels. 



The arachnoid membrane, as in the cord, is separated from the dura 

 by a cleft-like sub-dural space. In certain places, especially along the 

 sides of the superior sagittal sinus, there are found arachnoid villi (Pac- 

 chionian bodies or granulations), which project into the cavity of the ven- 

 ous sinus. They are elevations of the arachnoid covered with a thin por- 

 tion of the dura and venous endothelium, and possibly facilitate the trans- 

 fer of fluid between the arachnoid (or subarachnoid) spaces and the veins. 

 These spaces contain the cerebro-spinal fluid, and are continuous with 

 the corresponding spaces around the cord. Through apertures in the 

 thin roof of the fourth ventricle, they communicate with the central 

 cavity of the cord and brain. 



The pia is a delicate and highly vascular layer, containing arteries 

 which send branches into the cortex from all points on its surface. These 

 cortical arteries arise from the anastomoses between the internal carotid 

 and vertebral arteries at the base of the brain, which produce the arterial 

 circle of Willis. Other branches from these vessels enter the substance of 

 the base of the brain, supplying the basal nuclei, thalamus and internal 

 capsule. Because of the effects of haemorrhage in relation with the motor 

 and sensory tracts in this region, these small arteries are of very great 

 importance. The vascular membranes which cover the thin portions of 

 the roof of the third and fourth ventricles are in places invaginated into 

 the ventricles, forming the chorioid plexuses. These networks of small 

 vessels, covered only by thin membranes, are found in the lateral ven- 

 tricles, as well as in the third and fourth; their position is described in 

 text-books of gross anatomy. The simple layer of cuboidal epithelium, 

 which covers the plexuses, contains pigment granules and fat droplets, 

 and may perform secretory functions. 



EYE. 



Development and General Anatomy. The eyes first appear as a pair of 

 optic vesicles, which are lateral out-pocketings of the fore-brain (Fig. 

 45 1^ A). They enlarge rapidly, but their connections with the wall of 

 the brain remain relatively slender, forming the optic stalks. The epi- 

 dermal ectoderm immediately overlying the vesicles thickens and be- 

 comes invaginated (Fig. 451, B and C). The invaginated portion is then 

 detached in the form of a vesicle, the inner wall of which is distinctly 

 thicker than the outer; this "lentic vesicle" becomes the lens of the eye. 

 Meanwhile, as seen in B and C, that layer of the optic vesicle which is 



