THE MEMBRANES OF THE BRAIN. 



1203 



Vascular tuft 



fact that the apparent ingrowth of the pia beneath the splenium and the fornix to 

 reach its final position over the third and within the lateral ventricles never occurs, 

 the growth actually taking place in the opposite direction, that is, from before 

 backward (page 1194). 



The structure of the pia mater presents little for special mention. The 

 membrane consists essentially of a delicate connective tissue envelope in which inter- 

 lacing bundles of white fibrous tissue, intermingled with elastic fibres and containing 

 numerous nuclei, are the chief features. As the arteries leave the pia to enter 

 the brain, they receive sheaths of pial tissue within which are prolonged the lymph- 

 spaces enclosed between the trabeculae of the pial membrane. Along the basal surface 

 of the brain, especially on the ventral aspect of the medulla, the pia frequently contains 

 deeply pigmented branched connective tissue cells. These may be so numerous, 

 particularly in aged subjects, that the membrane appears of a distinct brownish hue. 



The numerous nerves encountered within the pia mater are chiefly sympathetic 

 filaments destined for the walls of the blood-vessels and derived from the plexuses 

 surrounding the internal carotid and the vertebral arteries. Additional nerve- 

 fibres, probably sensory in function, occur 



in small numbers. The mode of their FIG. 1037. 



ending is uncertain, although terminal bul- 

 bus expansions and tactile corpuscles have 

 been observed. 



The Arachnoid. This covering 

 (arachnoidea encephali), the intermediate 

 membrane of the brain, is a delicate con- 

 nective tissue envelope that intervenes 

 between the dura externally and the pia 

 internally. In contrast to the last-named 

 membrane, which follows closely all the 

 irregularities of the sunken as well as .of the 

 free surface of the cerebrum, the arachnoid 

 is intimately related to the convolutions 

 only along their convexities, and on arriving 

 at the margins of the intervening fissures 

 stretches across these furrows to the con- 

 volutions beyond. From this arrangement 

 it follows that intervals, more or less tri- 

 angular on section, are left over the lines 

 of the fissures between the arachnoid and 

 the fold of pia which dips into the sulcus. 

 These clefts form a system of intercom- 

 municating channels which are parts of the 



general subarachnoid space. Over the summits of the convolutions, the arachnoid 

 and pia are so intimately united that they constitute practically a single membrane, 

 whilst, where parted by the subarachnoid space, they are connected only by the 

 trabeculae of arachnoid tissue. In many places, however, where the intervening cleft 

 is not wide, these trabeculae are so numerous that the space is occupied by a delicate 

 reticulum and becomes converted into a layer of loose subarachnoid tissue. Where, 

 on the other hand, the arachnoid encloses spaces of considerable size, as it does on 

 the basal surface of the brain, the trabeculae are reduced in number to relatively few- 

 long, cobweb-like threads that extend from the arachnoid to the pia mater. Over 

 the upper and outer aspects of the cerebrum and cerebellum the arachnoid follows, 

 in a general way, the contour of the brain. On the ventral surface, however, it 

 bridges from the median elevation presented by the brain-stem to the adjacent promi- 

 nences offered by the cerebellum and the cerebral hemispheres. The irregular spaces 

 thus enclosed contain considerable quantities of cerebro-spinal fluid and are known 

 as the cisternae subarachnoidales, of which several subdivisions are recognized 

 according to locality. 



The cisterna magna (cisterna cerebellomedullaris), the largest of these spaces, 

 overlies the dorsal surface of the brain-stem and is continuous through the foramen 



Velum inter- 



positum 



Small portion of injected choroid plexus of lateral 

 ventricle; surface view. 



