Chapter III

are comprehended. Their method of activity is there discussed at sufficient length, and we need here only consider the various _paths of infection_. These may lie along the _bloodvessels_, the _lymphvessels_, _nerve sheaths_, and prolongations of the _membranous sacs_ which extend from the cranial cavity proper.

The most common of all the paths of infection is afforded by the _middle ear_, especially when involved in a chronic suppurative lesion, which is by no means necessarily connected with a patulous tympanic membrane, and which may consequently be undiscovered, though in more or less constant activity.

A. =Abscess of the Brain.=--This may be _traumatic_ or _non-traumatic_. The former variety is most often due to the direct result of injury, infection displaying its consequences promptly or sometimes not until long periods have elapsed. The ordinary form occurs within the first two weeks, usually as an acute cortical abscess beneath a more or less compromised membrane, surrounded by a zone of red softening, and this by another of brain edema. The _chronic_ traumatic abscesses are less often cortical, but are deeper. They are marked by prolonged suppuration of the external wound, but may occur through some mechanism not understood. Only the chronic abscesses show _encapsulation_, the capsule partaking of the character of the pyophylactic membrane, elsewhere described. (See