THE NERVOUS SYSTEM. 821 



brain tissue. The walls of the cavity are ragged and infiltrated with 

 pus, and outside of the walls is a zone of O3dematous and softened brain 

 tissue. If the abscess be near the pia mater, meningitis may follow ; if 

 it be near the lateral ventricles, it may rupture into them ; if it be near 

 the sinuses of the dura mater, it may induce thrombosis. An abscess 

 may in time become enclosed in a capsule of connective tissue. 



Abscess of the brain is most frequently secondary to chronic suppu- 

 rative otitis (42.5 per cent, Gowers), much less frequently to acute otitis. 

 With the otitis there may also be caries of the temporal bone, suppura- 

 tion of the mastoid cells, and inflammation of the dura mater. The 

 abscess is usually situated deep in the brain, commonly in the temporo- 

 sphenoidal, the frontal, the occipital or the parietal lobes, or in the 

 cerebellum; rarely it is continuous with the inflamed dura mater and 

 bone. Abscess may follow chronic lesions in the orbit or caries of vari- 

 ous parts of the cranial bones. 



Abscess of the brain frequently follows traumatism, blows, or falls 

 on the head. Such injuries may not damage the skull, or may produce 

 fractures or necrosis. There is often a considerable interval between 

 the time when the injury is inflicted and the development of the symp- 

 toms. 



When the cranial bones are uninjured the abscess is usually deep in 

 the brain ; when there is necrosis of the bones the abscess may be super- 

 ficial ; when the bones are fractured the abscess may be either superficial 

 or deep. The abscess develops rarely in the opposite side of the brain. 



In acute exudative meningitis from various excitants there may be an 

 infiltration of the brain with leucocytes ; this may be especially marked 

 in the perivascular tissue and around the ganglion cells. 



An Acute Disseminated Encephalitis, ' haematogenous in character, may 

 develop during the progress of an infectious disease. It is most com- 

 mon in infective endocarditis, in pyaemia, and in epidemic cerebrospiual 

 meningitis. It may be associated with acute anteropoliomyelitis, ap- 

 parently due to the same obscure etiological factor. The lesion consists 

 in disseminated foci of inflammation, or minute multiple abscesses. 

 Some of these are microscopic in size, others may be seen with the naked 

 eye. The smallest show simply sniall-round-cell infiltration of the walls 

 of one or more small vessels and of the surrounding tissue. The larger 

 spots, which undoubtedly take origin in the same way, are seen to be 

 softer than the rest of the tissue, and resemble red or yellow softening, 

 according to the amount of red blood cell extravasation. Congestion is 

 usually marked. There may be distinct haemorrhages. Degenerative 

 changes with disintegration of the exudate usually set in and determine 

 destructive changes in the neighboring neurone and ueuroglia elements. 

 At the periphery of one of these abscesses the neuroglia, instead of being 

 in a degenerating condition, usually shows proliferation. In the case of 



1 Under the head of " Acute Parenchymatous Encephalitis " has been described a 

 lesion of the ganglion cells without vascular or interstitial changes. These lesions are 

 the result of toxaemias of either endogenous or exogenous nature. They are more prop- 

 erly classed as parenchymatous degeneration. 



