PRACTICAL CONSIDERATIONS : THE BRAIN. 1209 



operation, indicates a previous inflammation, as does any tendency of the arachnoid 

 to adhere to the dura, since these two are normally not adherent. The arachnoid, 

 however, is normally closely attached to the pia, and for practical purposes they 

 are usually considered as one layer, the lepto-meninx. 



Inflammation of this layer lepto-meningitis may attack the convexity or the 

 base of the brain, and may be primary or may be secondary to other diseases, usually 

 purulent infections. It is asserted that the primary disease attacks, as a rule, the 

 base, the secondary, the convexity of the brain ; but this is not beyond dispute. 



Tuberculous meningitis is frequently found at the base, but miliary tubercles are 

 not uncommon on the convexity of the brain. The exudate which is deposited at 

 the base frequently leads to irritation or paralysis from pressure on the cranial nerves 

 in close relation to the under surface of the brain. Tumors growing at the base of 

 the brain produce localizing symptoms early by pressing on the adjacent cranial 

 nerves. A single nerve may be involved, but more commonly a combined paralysis 

 from involvement of several nerves results. 



The cerebro-spinal fluid is found in the subdural and subarachnoid spaces, and 

 in the ventricles. Over the vault it is comparatively scanty in both spaces. At the 

 base, however, in the subarachnoid space of the middle and posterior fossae, it is 

 abundant, forming an excellent support and protection to the most delicate part of 

 the brain, that containing the vital centres. The frontal lobes, of much less impor- 

 tance as to vital function, rest directly on the bone in the anterior fossa ; and are there- 

 fore more subject to direct traumatic influences. The fact that the subarachnoid 

 space is continuous with the ventricles through the foramina of Magendie and of 

 Luschka, and communicates freely at the foramen magnum with the subarachnoid 

 space of the cord, explains how excess of pressure within the cranium at one part 

 may be relieved by escape of fluid to other parts. It explains also why pressure 

 on a spina bifida will sometimes produce symptoms of cerebral compression ; and 

 vice versa, why the increased congestion of the cerebral vessels from expiratory 

 efforts, as in coughing, will increase the tension in the spinal tumor. 



Occlusion of the foramen of Magendie, by the products of inflammation, may 

 cause increase of fluid from retention in the ventricles, with the development of 

 hydro cephalus, and it is in this way that internal hydrocephalus occasionally follows 

 meningitis. For the purpose of determining the cause of this condition, subarach- 

 noid fluid is sometimes withdrawn through a hollow needle. 



The lateral ventricles can be tapped through a trephine opening 3 cm. (i^ 

 in.) behind the external auditory meatus, and the same distance above Reid's base 

 line drawn from the lower margin of the orbit through the middle of the external 

 auditory meatus. The needle is passed towards a point on the opposite side of the 

 skull, 6.5-7.5 cm. (2^-3 in.) vertically above the external auditory meatus. Under 

 normal circumstances the ventricle is from 5-5.6 cm. (2-2^ in.) from the surface, 

 but if the ventricle is distended the distance is shorter. 



By a trephine opening in the occipital bone in the subcerebellar region, the 

 subarachnoid fluid has been reached at the base of the brain where it is most 

 abundant. 



Lumbar puncture for withdrawing cerebro-spinal fluid for diagnostic and thera- 

 peutic purposes is sometimes employed. The needle should be introduced between 

 the third and fourth, or between the fourth and fifth lumbar vertebrae, at the level 

 of the lower border of the spinous process, or opposite its lower third, and about 

 I cm. from the median line. It should be passed somewhat upward between the 

 sloping laminae, and should be continued inward toward the canal until, by the 

 diminished resistance, it is recognized that the point of the needle has entered the 

 subarachnoid space. 



The Brain. Of all the affections of the brain, hemorrhage is the most frequent 

 and most important, whilst in the spinal cord it is comparatively rare unless as a 

 result of trauma. Hemorrhage from the meningeal vessels is most commonly due to 

 trauma, but within the brain substance the usual cause is atheroma, sometimes with 

 the production of miliary aneurisms. A sudden strain increases the intravascular 

 tension and ruptures one of these diseased vessels, giving rise to pressure symptoms, 

 depending on the seat and extent of the hemorrhage. 



