Pathogenesis. Anatoniieal Changes. 599 



especially in the membranes, by a cellnlar infiltration. The dis- 

 covery made by Dexler in a case of the socalled Borna disease 

 associated with the investigations made in hnman medicine 

 (Sclmltze) indicate that when the inflammation is not snppnra- 

 tive it may affect the deeper portions of the cortex in some 

 cases and the snmmits of the gyri in others. This affects the 

 macroscopic characters of the lesions, as in the latter case the 

 alterations are clearly visible to the naked eye. Chemical sub- 

 stances which are directly injurious to the nervous tissue, and 

 the effect of which on the blood vessels has started the inflamma- 

 torv process in association with the increased intracranial pres- 

 sure due to hyperemia and extravasation, cause a series of 

 symptoms indicative of cerebral disturbance. In view of the 

 fact that the lymph spaces of the subarachnoid are in com- 

 munication with each other and with those of the spinal cord the 

 inflammation easily spreads to the whole of the surface of the 

 brain and generally extends to a less degree to the meninges of 

 the cord. The roots of the cranial nerves passing through the 

 membranes are very often involved. The stimulation of the 

 nerve endings in the membranes due to the inflammation and 

 the increased intracranial pressure cause pains in the head. 



Anatomical Changes. The acute inflammation may in cer- 

 tain cases remain localized though, as a rule, it tends to become 

 diffuse. The congestion of the vessels may be so slight that a 

 definite diagnosis can only be arrived at by means of micro- 

 scopic examination. In tlie great majority of cases there is an 

 accumulation of more or less turbid exudate which may be 

 either colorless or reddish in between the dura and the arach- 

 noid and sometimes under the pia mater (hydrocephalus ex- 

 ternus). Far more rarely the exudate is fibrinous and sur- 

 rounds the dilated blood vessels, making them appear as yel- 

 lowish-white streaks. In cases of diffuse meningitis all these 

 lesions are found, involving also the pons and medulla and to a 

 less extent the membranes of the spinal cord. The lesions may 

 be particularly prominent in the anterior portions of the hem- 

 ispheres in cases of primary meningitis in cattle (Poulsen) . As 

 a general rule pus tends to collect at the places where the in- 

 flammation started, causing the pia mater to appear as a thick 

 membrane saturated with pus, Avhile round about there is evi- 

 dence of hyperemia and serous inflammation only (purulent 

 meningitis). In cases of tuberculous meningitis there are, in 

 addition to purulent and gelatinous infiltration of the meninges, 

 grayish translucent or yellow tubercles varying in size from a 

 poppy seed to a hemp seed along the course of the larger vessels 

 at the base of the brain. 



The cortex and the contiguous white matter appear sat- 

 urated with serous liquid, moist, and swollen, and in conse- 

 quence of this the convolutions appear less prominent. On the 

 cut surface are visible vessels extending inwards from the pia 



