THE NERVOUS SYSTEM. 767 



number. The frequency of their occurrence, and the absence of any known pathologi- 

 cal significance, warrant our .regarding them as essentially normal structures. 



The pia mater is frequently thickened, opaque, and white, either in diffuse patches 

 or, more commonly, along the course of the vessels. In other cases single or multiple 

 small white spots, of the size of a pin's head or smaller, may be seen in the membrane, 

 not appreciably elevated above the surface, but due to localized thickening. These 

 slight opacities of the pia mater are commonly believed to be dependent upon repeated 

 congestions of the membrane or upon chronic meningitis, but there is no evidence that 

 this is always the case. They are most frequently found in old persons, but may exist 

 at any age, and do not necessarily indicate the pre-existence of disease. 



The amount of blood contained in the vessels of the pia mater after death varies 

 greatly, and is by no means a reliable indication of the amount present during life. In 

 general anaemia the vessels of the pia mater may contain little blood, but, on the other 

 hand, they sometimes seem to contain a relatively larger amount than other parts of the 

 body. In oedema of the brain and pia mater the vessels of the latter may contain but a 

 small amount of blood. 



(EDEMA. 



The quantity of serum beneath the pia mater and infiltrating its tis- 

 sue is very variable in amount. It may accumulate as a result of atrophy 

 of the brain substance or of venous hypersemia, and sometimes is, and 

 sometimes is not, accompanied by redema of the brain substance. It 

 may be diffuse or localized. It is not infrequent to find in hospital pa- 

 tients suffering from chronic nephritis, cardiac or pulmonary disease, or 

 chronic alcoholism, a very considerable amount of serum in this situa- 

 tion, and yet the patient has been free from cerebral symptoms. In 

 other cases a serous effusion may accompany grave cerebral symptoms. 

 It is necessary to be very careful in judging of the importance of this 

 accumulation of fluid, especially in determining the cause of death in 

 the absence of other marked lesions (see page 789). 



It should always be borne in mind that an accumulation of fluid be- 

 neath the pia mater and in its meshes may occur as a result of post- 

 mortem changes. 



HYPEREMIA AND HEMORRHAGE. 



Hypersemia. The pia mater may be hypera3inic in early stages of men- 

 ingitis, in delirium tremens, in epileptic convulsions, in infectious dis- 

 eases, and in poisoning. It may be due to the pressure of exudates or 

 tumors on the veins, or to general or local lesions of the circulatory 

 system. The time which elapsed between death and the autopsy, the 

 position in which the body has lain, the coagulability of the blood, may 

 have an important bearing upon the amount and situation of blood in 

 the pia. 



Haemorrhage. This may occur either into the space between the dura 

 mater and the pia mater intermeningeal hcemorrhage or in the meshes of 

 the pia or between the latter and the brain. It may be due to injury, 

 to rupture of aneurisms or otherwise diseased blood-vessels, to throm 

 boses of the venous sinuses, or to conditions which we are unable to as- 



