This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
This, when not of a tuberculous character is called leptomeningitis. It is commonly known as inflammation of the brain, or meningitis. The pia mater of the brain being directly continuous with that of the spinal cord, inflammations of the former extend to and involve the latter in about one-third of the cases. The disease is then called cerebrospinal meningitis.
Infection is the usual cause of leptomeningitis. Direct injury to the membranes and their bony envelopes may cause it, but it occurs usually through some secondary avenue of infection. Thus, it may follow fractures opening into the mouth, nose, the various accessory bony sinuses, ear, etc. The infection may, however, not. be traumatic, but occur through the blood, following or accompanying the various infectious diseases. Owing to the fact of the pia lying on the brain substance, and its vessels with their perivascular sheaths penetrating it, the disease naturally tends to invoke the brain, if it is very severe or long standing. If such is the case, the affection is called cerebritis or encephalitis.
The inflammation may be serous, plastic, or even purulent. The pia mater being continuous with the choroid plexuses, the ventricles may be dilated by the increased fluid. The infection may follow the vessels into the brain and produce brain abscess. The effusion being often localized at the base of the brain interferes with the functions of the cerebral nerves. The first, or olfactory, is comparatively rarely affected. The optic, or second, is more often so, producing intolerance of light. There may be choked disk, and I have even seen a case in which there was total blindness without any change being visible in the nerve by means of ophthalmoscopy. In this case atrophy of the disk soon followed. The third, or motor oculi, according to Church and Peterson, is almost always affected. This would be shown by strabismus, diplopia, and changes in the pupil. Facial paralysis, from implication of the seventh nerve, is sometimes seen, and the auditory, or eighth, may also be affected. Involvement of the hypoglossal or twelfth nerve, will be shown by deviation of the tongue. The origin of the cranial nerves from the base of the brain is shown in Fig. 28.
In this form of meningitis the infection comes through the blood, and the tuberculous lesions follow the vessels. They are most marked on the base of the brain, involving the circle of Willis and the Sylvian fissure. The infection follows the vessels of the pia mater through the transverse fissure into the ventricles. The effusion accumulating in the ventricles has given rise to the name acute hydrocephalus. It also follows the perivascular sheaths of the smaller vessels into the brain substance, producing a cerebritis; thus it is seen how a knowledge of the circulation of the brain explains the location of the lesions.
The exudate involving the nerves of the base of the brain produces corresponding symptoms by interfering with their function.