If such is the case--and I think that refutation is impossible--and if it be accepted that definite pathological changes are usually present, then it is obvious that surgical interference offers the only hope of cure or alleviation. The surgeon must be guided by the case before him, but, in the absence of hereditary taint, exploration should be carried out, preferably at the site of the original injury, and as early as possible before the advent of definite cerebral and pyramidal degenerations. The operative details vary according to the necessities of the case. Thus, in acute mania, acting on the supposition that the development is dependent on an acute cerebral œdema, the surgeon should carry out a decompression operation. Personally, I hold the view that Cushing’s subtemporal decompression operation (see p. 121) should be carried out on both sides of the skull, with or without lumbar puncture. In the more chronic cases, operative procedures are conducted over the region injured in the anticipation of discovering depressed bone, subdural cysts, hæmatomata, &c. The operative details peculiar to all of these conditions are discussed in other sections. With special reference to general paralysis of the insane, Dr. Claye Shaw holds the opinion that a general increase of intracranial pressure is commonly present. At his instigation, and on other occasions, I have carried out decompression operations, but I have not been able to satisfy myself that the patients have experienced any benefit other than temporary. There was on each occasion a considerable excess of cerebro-spinal fluid with surface œdema of the brain, but the ultimate results were certainly disappointing. MENINGEAL CYSTS Amongst the pathological conditions responsible for the development of epilepsy, chronic cephalalgia, &c., meningeal cysts must be regarded as of frequent occurrence. Traumatic meningeal cysts are varied in size, site, and structure. They may be classified as follows:-- 1. _Cysts within the calvarium._ Between the dura and the bone. Between the dura and the brain. Within the brain-substance. 2. _Cysts projecting through a gap in the calvarium_ (traumatic cephaloceles). For a description of these cysts the reader is referred to p. 40. =Cysts within the calvarium.= Traumatic cysts within the calvarium, whatever their situation, may be divided into two definite groups:-- 1. Where the cyst possesses fibrinous walls (in which hæmatoidin cysts may be deposited) and contains a grumous material. These cysts are usually subdural in position, are dependent on the previous existence of a hæmatoma, and are known as ‘blood-cysts’.