THE SPINAL CORD AND ITS MEMBRANES. 707 



and 3f inches 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 Professor Thane gives the lower end of the furrow as 

 "close to the posterior limb, and about half an inch behind the bifurcation of the fissure of 

 Sylvius." So that, according to this anatomist, a line drawn from a point half an inch behind 

 the mid-point between the glabella and external occipital protuberance to this spot would mark 

 out the fissure of Rolando. Dr. Reid adopts a different method (Fig. 376). He first indicates 

 on the surface the longitudinal fissure and the horizontal limb of the fissure of Sylvius (as above). 

 He then draws two perpendicular lines from his "base-line" (that is, a line from the lowest 

 part of the infra-orbital margin through the middle of the external auditory meatus to the back 

 of the head) to the top of the cranium, one (D K, Fig. 376) from the depression in front of the 

 external auditory meatus, and the other (F G, Fig. 376) from the posterior border of the mastoid 

 process at its root. He has thus described on the surface of the head a four-sided figure (F D G E, 

 Fig. 376), and a diagonal line from the posterior superior angle to the anterior perpendicular 

 line where it is crossed by the fissure of Sylvius will represent the furrow. 



The parieto-ocamtal fissure on the upper surface of the cerebrum runs outward at right 

 angles to the great longitudinal fissure for about an inch, from a point one-fifth of an inch in 

 front of the lambda (posterior fontanelle). Reid states that if the horizontal limb of the fissure 

 of Sylvius be continued onward to the sagittal suture, the last inch of this line will indicate the 

 position of the sulcus. 



The j'ti-i'ci'.iitrtil xidciis begins four-fifths of an inch in front of the middle of the fissure of 

 Rolando, and extends nearly, but not quite, to the horizontal limb of the fissure of Sylvius. 



The superior frontal fissure runs backward from the supra-orbital notch, parallel with the 

 line of the longitudinal fissure to two-fifths of an inch in front of the line indicating the position 

 of the fissure of Rolando. 



The inferior frontal fosure follows the course of the superior temporal ridge on the frontal 

 bone. 



The intraparietal fissure begins on a level with the junction of the middle and lower third 

 of the fissure of Rolando, on a line carried across the head from the back of the root of one 

 auricle to that of the other. After passing upward it curves backward, lying parallel to the 

 longitudinal fissure, midway between it and the parietal eminence ; it then curves downward to 

 end midway between the posterior fontanelle and the parietal eminence. 



THE SPINAL CORD AND ITS MEMBRANES. 



Dissection. To dissect the cord and its membranes it will be necessary to lay open the 

 whole length of the spinal canal. For this purpose the muscles must be separated from the 

 vertebral grooves, so as to expose the spinous processes and laminae of the vertebrae ; and the 

 latter must be sawn through on each side, close to the roots of the transverse processes, from 

 the third or fourth cervical vertebra above to the sacrum below. The vertebral arches having 

 been displaced by means of a chisel and the separate fragments removed, the dura mater will be 

 exposed, covered by a plexus of veins and a quantity of loose areolar tissue, often infiltrated with 

 serous fluid. The arches of the upper vertebrae are best divided by means of a strong pair of 

 cutting bone-forceps. 



MEMBRANES OF THE CORD. 



The membranes which envelop the spinal cord are three in number. The most 

 external is the dura mater, a strong fibrous membrane which forms a loose sheath 

 around the cord. The most internal is the pia mater, a cellulo-vascular membrane 

 which closely invests the entire surface of the cord. Between the two is the 

 arachnoid membrane, a non-vascular membrane which envelops the cord and is 

 connected to the pia mater by slender filaments of connective tissue. 



The Dura Mater of the cord represents only the meningeal or supporting layer 

 of the cranial dura mater. The endocranial or endosteal layer ceases at the foramen 

 magnum posteriorly, but reaches as low as the third cervical vertebra in front ; 

 below these levels its place is taken by the periosteum. It forms a loose sheath 

 which surrounds the cord, and is separated from the bony walls of the spinal canal 

 by a quantity of loose areolar tissue and a plexus of veins. The situation of the 

 veins between the dura mater of the cord and the periosteum of the vertebrae 

 corresponds therefore to that of the cranial sinuses between the endocranial and 

 supporting layers. It is attached to the circumference of the foramen magnum, 

 and to the axis and third cervical vertebra ; it is also fixed to the posterior common 

 ligament, especially near the lower end of the spinal canal, by fibrous slips ; it 

 extends below as far as the second or third piece of the sacrum ; here it becomes 



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



