768 THE NERVOUS SYSTEM. 



certain. Haemorrhages without known cause not infrequently occur in 

 the substance of the pia mater in young children, but in adults they are 

 apt to be the result of injury. Multiple ecchymoses, however, in the 

 substance of the pia mater sometimes occur in infectious diseases and 

 also in acute inflammation of the pia mater. Haemorrhages in the brain 

 substance may lead to the accumulation of blood beneath or in the 

 meshes of the pia mater. Intermeniugeal haemorrhage in infants as a 

 result of injury during birth is not uncommon. Small, and sometimes 

 considerable, extravasations of blood may occur from diapedesis, and 

 sometimes, as a result of chronic congestion, degenerated blood pigment 

 collects along the walls of the vessels. The extravasated blood in nieniu- 

 geal haemorrhage, if small in quantity, may be largely absorbed, leaving 

 a greater or smaller accumulation of pigment at the seat of the haemor- 

 rhage. Such pigmentations may last for a long time. 



INFLAMMATION. (Meningitis Leptomeningitis. ) 



We distinguish acute, chronic, tuberculous, and syphilitic meningitis. 



Acute Meningitis may occur as the characteristic lesion of epidemic 

 cerebro- spinal meningitis ; it is a not very infrequent complication of 

 pneumonia, typhus and typhoid fever, the exanthemata, and of chronic 

 diffuse nephritis; it may be secondary to injuries and inflammation of 

 the cranial bones, of the dura mater, and of the middle ear, and it some- 

 times occurs as an independent infectious process. 



In acute meningitis the inflammatory process is apt to extend down- 

 ward and involve the pia mater of the cord. It may also involve the 

 ependyma of the ventricles, and cause the distention of these cavi- 

 ties with serum. This latter condition is especially frequent in young 

 children. 



It is convenient to consider two varieties of acute meningitis, one in 

 which there is cell proliferation with little or no exudate, the other in 

 which exudate is present. In many cases at least the first may be the 

 early stage of the latter form. 



1. ACUTE CELLULAR MENINGITIS. The pia mater is congested, its 

 surface is dry and lustreless, and it is somewhat opaque. The changes 

 in the gross appearance of the membrane are not marked and are easily 

 overlooked, but the minute changes are more decided. There is a pro- 

 liferation of the eudothelial and connective-tissue cells of the pia, and 

 often a moderate collection of fluid and leucocytes in the meshes of the 

 pia. This cell proliferation may involve the pia mater over most of the 

 surface of the brain. This form of meningitis is of frequent occurrence 

 and is attended with the ordinary clinical symptoms of acute meningitis. 



2. ACUTE EXUDATIVE MENINGITIS is characterized by the accumu- 

 lation, chiefly in the meshes of the pia mater and along the walls of the 

 blood-vessels, of variable quantities of serum, fibrin, and pus. Some- 

 times one, sometimes another of these preponderates, giving rise to ser- 

 ous, fibrinous, or purulent forms of the inflammation. The absolute 



