A rubber drainage-tube may be inserted in the manner depicted in the same figure. Except at the point of emergence of the tube, the dura mater is sewn up (with fine interrupted catgut sutures). (_B_) =Localized subdural hæmorrhage (subdural hæmatocele).= We are greatly indebted to Bowen[31] for our knowledge of subdural hæmatomata. That subdural hæmorrhage might take the form of a localized collection was a recognized fact, but the condition was but little understood, the typical clinical symptoms were not recognized, and surgeons hesitated to adopt surgical remedies. All these facts impeded advance in both diagnosis and treatment. Subdural hæmatoceles, even at this date, are often regarded as rarities, but, in the light of present knowledge, it would appear that wider recognition of the hæmorrhage in its clinical aspect will show that such ideas are erroneous. In the preceding section it was shown that subdural extravasation resulting from sinus-injury tends to become diffused throughout the subdural space. With respect to localized hæmorrhages our data are by no means complete, but all available evidence tends to show that subdural hæmatoceles are dependent on laceration of the pia-arachnoid vessels--a condition practically synonymous with superficial laceration and contusion of the cortex. The blood, derived from small cerebral veins and minute cerebral arteries, exercises primarily but slight pressure effect. Its force is expended on compressing and emptying the subjacent and surrounding cerebral vessels, producing, in other words, a condition of _local cerebral anæmia_. This pressure is insufficient to lead to the development of general compression, but suffices to produce certain rather indefinite symptoms. We have, therefore, before us a very different picture to that depicted in cases of middle meningeal hæmorrhage. In such extra-dural hæmorrhages some definite period of time must elapse before the dura mater is sufficiently stripped from the bone to allow of the formation of a clot of size sufficing to exercise both local and general compression effects. Pressure effects then become very manifest. On the other hand, in localized subdural extravasations there is an immediate effect, for the brain is contused or lacerated. On recovering from the immediate effects of the injury--concussion of a greater or lesser degree of intensity--the patient does not regain complete mental and bodily convalescence. He suffers from symptoms suggestive of brain irritation--headaches, photophobia, mental irritability, insomnia, loss of appetite, the pulse accelerated, and the temperature slightly raised. Later on, more definite symptoms arise, but, intervening between the day of the accident and the time at which these more definite localizing symptoms develop, there is an unmistakable _latent_ period--a period to be sharply differentiated from the previously mentioned _lucid_ interval (see p. 139).