966 THE NERVE SYSTEM 



the shorter, oblique strip will indicate the direction and 9 to 10 cm. will mark the length of the 

 furrow. Dr. Wilson has devised an instrument, called a cyrtometer, which combines the scale 

 of measurements for localizing the fissure with data for representing its length and direction. 1 



The Occipital Fissure on the dorsal surface of the cerebrum runs outward at right angles to the 

 great longitudinal fissure for about 2 to 3 cm. (f to 1} inches), from a point 0.5 cm. ( inch) 

 in front of the lambda. Reid states that if the sylvian line be continued onward to the sagittal 

 suture, the last 2 to 3 cm. of this line will indicate the position of the fissure. 



The Precentral Fissure begins 2 cm. ($ inch) in front of the middle of the central fissure and 

 extends nearly, but not quite, to the sylvian fissure. 



The Superf rental Fissure runs backward from the supraorbital notch, parallel with the line 

 of the longitudinal fissure to 1 cm. (f inch) in front of the line indicating the position of the 

 central fissure. 



The Subfrontal Fissure follows the course of the supertemporal ridge on the frontal bone. 



The Interparietal Fissure, comprising the parietal, subcentral, and paroccipital fissures, begins 

 on a level with the junction of the middle and lower third of the central fissure, on a line carried 

 across the head from the back of the root of one auricle to that of the other. After passing up- 

 ward it curves backward, lying parallel to the longitudinal fissure, midway between it and the 

 parietal eminence; then curves downward to end midway between the posterior fontanelle and 

 the parietal eminence. 



Kronlein's method for determining the portions of certain fissures of the brain is very useful 

 and easy of application (Fig. 716). It is as follows' (1) The base line, Z M, is a horizontal 

 line running at the level of the lower border of the orbit and the upper border of the external 

 auditory meatus. (2) Another horizontal line, K K', is drawn parallel to Z M. The second 

 horizontal line is on a level with the supraorbital ridge. (3) A vertical line, Z K, is erected 

 from Z M at the middle of the zygoma and is carried to the line K K'. (4) Another vertical line, 

 A R, is erected from the base line at the level of the articulation of the mandible and is carried 

 to R. (5) A third vertical line, M P, is erected from the base line at the posterior border of the 

 mastoid process and is carried to the middle line of the skull, which is marked P. (6) A line is 

 drawn from K to P. The portion of this line between R and P' corresponds to the central 

 fissure. (7) The angle P K K' is bisected by the line K S. K S corresponds to the sylvian fis- 

 sure, and K is directly over the sylvian point. To reach the anterior branch of the middle 

 meningeal, apply the trephine at K; to reach the posterior branch, apply it at K'. In abscess 

 of the temporal lobe the trephine should be applied, according to von Bergmann, in the region 

 AaK M. 



THE MENINGES, OR MENINGEAL MEMBRANES OF THE BRAIN 

 (MENINGES ENCEPHALI). 



Dissection. To examine the brain with its membranes, the skullcap must be removed. 

 In order to effect this, saw through the external table, the section commencing, in front, about 

 2 cm. (| inch) above the margin of the orbits, and extending, behind, to a little above the level of 

 the occipital protuberance. Then break the internal table with the chisel and hammer, to avoid 

 injuring the investing membranes or brain; loosen and forcibly detach the skullcap, and the 

 dura will be exposed. The adhesion between the bone and the dura often is very intimate, 

 particularly along the sutures. 



The membranes of the brain are from without inward the dura, arachnoid, 

 and the pia. 



The Dura of the Brain (Dura Mater Encephali) (Figs. 717, 722). 



The dura of the brain is a thick and dense, inelastic, fibrous membrane 

 which lines the interior of the skull. It is a covering for the brain and is also 

 the internal cranial periosteum. It is composed of two layers closely connected, 

 except in Certain situations, where, as already described (p. 723), they separate 

 to form sinuses for the passage of venous blood. Upon the outer surface of 

 the cranial dura, in the situation of the longitudinal sinus, may be seen numerous 

 small whitish bodies, the glandulae Pacchionii (granulationes arachnoideales). 

 Its outer surface is rough and fibrillated, and adheres closely to the inner surface 



i Lancet. 1888, vol. i, p. 408 



