Of the pia mater.—This membrane being the vascular membrane of the brain, and con taining the nutrient vessels as well of the sur face of the brain as of the visceral layer of the araehnoid, is the seat of all those changes in the condition of the bloodvessels or of their contents, which give rise to, or are caused by, morbid states either of the nervous matter or of the serous membrane.
All those changes which indicate hyperwmia or anwmia of the convolutions of the brain occur in the pia mater ; and the colour of this membrane will vary according to the quantity of blood contained in its bloodvessels.
There are no definite signs which enable the anatomist to pronounce whether an hypermmia be of the active and inflammatory kind, or passive, and dependent on some cause remote from the brain itself, or even upon a post inortem cause, unless it be accompanied with those undoubted products of the inflammatory process, pus or lymph.
A highly injected state of the vessels of the pia mater will frequently be caused by the manner of the patient's death. Where the respiratory actions have been laboured and dif ficult prior to death, this is sure to occur : we find it also when death has been caused by asphyxia, however produced.
In convulsive diseases the pia mater and the whole brain become highly injected more as a consequence of the impeded circulation caused by the struggles of the patient interfering with the due exercise of the respiratory movements, than as the cause of the convulsions. Indeed, there seem good grounds for believing that con vulsions are more frequently caused by a de ficient supply of blood to the brain than by a superabundant flow of it to that organ.
The pia mater is the seat of the principal morbid deposits which affect the brain. Of these tubercle is among the most common ; it most frequently occurs on the surface of the convolutions; but it may be found wherever the pia mater exists, either in the interior or on the outside of the brain. It occurs less fre quently on the pia mater of the cerebellum than in any other situation.
The tubercular deposit in the pia mater com mences by the developement of minute granu lations of a grey, clear, or semi-transparent material. These are deposited close to each other over a greater or less surface, forming a group, and several such groups may be formed near each other. After a time this grey ma
terial is changed into a yellow granular matter, which is sometimes enclosed in a cyst.
Tubercular matter originally deposied on the surface of the pia mater in the sulcus of a convolution may have the appearance as if it had been formed in the substance of the brain. The sulcus is obliterated, and the tubercle, enlarging towards the brain, becomes, in a short time, surrounded by cerebral matter.
Sometimes tubercle deposited in some part of the pia mater excites inflammation in the pia mater and arachnoid immediately near it,— tubercular meningitis; and this may affect more or less of the substance of the bmin in its vicinity, causing red softening.
Cerebral tubercle is seldom or never alone. Other organs of the body are almost invariably affected at the same time, the lymphatic or the mesenteric glands, or the lung,s. It is most commonly found in children, and it is not im probable that it may lie dormant for years until roused to action by some newly-developed morbid excitant.
Connected with diseased states of the mem branes of the brain, it should be remarked that in many instances acute affections of the mem branes of the brain find their point of departure in inflammation of the sinuses. The sinus which is most frequently inflamed is the lateral; the inflammatory state of this spreads to the neighbouring arachnoid and pia mater, and induces all the consequences of a primary me ningitis.
Of the abnormal states of the brain.—The abnormal conditions of the brain may be con sidered under the heads of-1, congenital ; 2, acquired or morbid.
1. Congenital abnormal conditions.—A total defect of the brain is found in that state in which the head is wanting (Acephalia); and also where there is deficiency of the parietal bones of the cranium, the occipital, temporal, sphe noid, and frontal being present in an imperfect state, and there being also, in general, spina bifida of the upper cervical vertebrx, there is a deficiency of a considerable portion of the en cephalon, the medulla oblongata or a portion of it being alone present (Anencephalia).