642 THE NERVOUS SYSTEM. 



the foramina of Key and Retzius. It is stated by Merkel that the lateral ven- 

 tricles also communicate with the subarachnoid space at the apices of their descend- 

 ing horns. 



The subdural space also contains fluid ; this is, however, small in quantity 

 compared with the cerebro-spinal fluid and is probably of the nature of lymph. 



The cerebro-spinal fluid fills up the subarachnoid space. It is a clear, limpid 

 fluid, having a saltish taste and a slightly alkaline reaction. According to 

 Lassaigne, it consists of 98.5 parts of water, the remaining 1.5 per cent, being 

 solid matters, animal and saline. It varies in quantity, being most abundant in 

 old persons, and is quickly reproduced. Its chief use is probably to afford 

 mechanical protection to the nervous centres, and to prevent the effects of 

 concussions communicated from without. 



Structure. The arachnoid consists of bundles of white fibrous and elastic 

 tissue intimately blended together. Its outer surface is covered with a layer of 

 endothelium. Vessels of considerable size, but few in number, and, according to 

 Bochdalek, a rich plexus of nerves derived from the motor division of the fifth, 

 the facial, and the spinal accessory nerves, are found in the arachnoid. 



Glandulae Pacchioni or Arachnoid Villi. 



The glandulae Pacchioni are numerous small whitish granulations usually col- 

 lected into clusters of variable size, which are found in the following situations : 

 1. Upon the outer surface of the dura mater, in the vicinity of the superior longi- 

 tudinal sinus, being received into little depressions on the inner surface of the 

 calvarium. 2. On the inner surface of the dura mater. 3. In the superior 

 longitudinal sinus. 4. On the pia mater, near the margin of the hemispheres. 



These bodies are not glandular in structure, but simply enlarged normal villi 

 of the arachnoid. In their growth they appear to perforate the dura mater, and 

 when of large size they cause absorption of the bone, and come to be lodged in 

 pits or depressions on the inner table of the skull. Their manner of growth is as 

 follows : at an early period they project through minute holes in the inner layer of 

 the dura mater, which open into large venous spaces situated in the tissues of the 

 membrane, on either side of the longitudinal sinus and communicating with it. 

 In their onward growth the villi push the outer layer of the dura mater before 

 them, and this forms over them a delicate membranous sheath. In structure they 

 consist of spongy trabecular tissue, covered over by a membrane, which is continu- 

 ous with the arachnoid. The space between these two coverings, derived from the 

 dura mater and arachnoid respectively, corresponds to and is continuous with the 

 subdural space. The spongy tissue of which they are composed is continuous 

 with the trabecular tissue of the subarachnoid space ; so that fluid injected into 

 the subarachnoid space finds its way into the Pacchionian bodies ; and through 

 their coverings filters into the superior longitudinal sinus. They are supposed to be 

 the means by which excess of cerebro-spinal fluid is got rid of, when its quantity 

 is increased above normal. 



These bodies are not found in infancy, and very rarely until the third year. 

 They are usually found after the seventh year ; and from this period they increase 

 in number as age advances. Occasionally they are wanting. 



The Pia Mater. 



The pia mater is a vascular membrane, and derives its blood from the internal 

 carotid and vertebral arteries. It consists of a minute plexus of blood-vessels, held 

 together by an extremely fine areolar tissue. It invests the entire surface of the 

 brain, dipping down between the convolutions and laminae, and is prolonged into the 

 interior, forming the velum interpositum and choroid plexuses of the lateral and fourth 

 ventricles. Upon the surfaces of the hemispheres, where it covers the gray matter of 

 the convolutions, it is very vascular, and gives off from its inner surface a multitude 

 of minute vessels, which extend perpendicularly for some distance into the cerebral 



