MENINGES G71 



Neuroglia is tlio name given to the peculiar interstitial tissue of the hrain and cord. 

 It is continuous with processes of pia mater which di}) in from the surface. 



Nerve-fibres are processes or outgrowths of the nerve-cells of the central 

 nervous system, from which they pass to he di.stril)uted to every part of the hody. 

 They are classed by physiologists into efferent and afferent fibres. Efferent "or 

 centrifugal nerve-fibres convey impulses from the nerve-centres; they comi)rise 

 motor nerves to muscles, secreto-motor nerves to glands and vaso-motor nerves. 

 Afferent or centripetal nerves convey impulses towards the nerve-centres; they 

 comprise sensory nerves and other nerves conveying impulses, which under ordinary 

 circumstances do not produce conscious sensations. Xerve-fibres run for the 

 greater part of their course in com})act bundles ui various sizes, and these bundles 

 are called nerves in the most general u.se of the term. Nerves are usually n)ixed 

 (i.e. contain Ijoth efferent and afferent fibres). Most nerves are of a whitish colour 

 owing to a medullary sheath which invests the essential parts (axis-cylinders) of 

 the individual fibres. Others (chiefly visceral nerves) are of a pearly-grey colour 

 owing to the absence of the medullary sheath in the majority of the fibres which 

 make up the nerve. 



The fibres of sensory nerves are outgrowths from nerve-cells contained in ganglia, such as the 

 Gasserian gangUon and the ganglia on the posterior roots of the sjiinal nerves. These ganglia, 

 although appearing to belong to the peripheral nervous system in the adult, were (in early 

 embryonic life) in continuity with the general epiblast of the neural canal. 



Certain terms which are in frequent use, such as coronal, sagittal, and hori- 

 zontal, may be defined here. A coronal section is a vertical section j^assing 

 through or parallel to the coronal suture, in other words, a transverse vertical 

 section. A sagittal section is a vertical section taken at right angles to a coronal 

 section — an antero-posterior vertical section. A horizontal section is a section 

 taken at right angles to the perpendicular axis of the Ixtdy. Structures which lie 

 in the planes of these sections are sometimes spoken of as coronally placed, 

 sagittally directed, etc. 



THE MENINGES 



The brain and spinal cord are invested by three membranes, termed the men- 

 inges, which afford protection and support to the delicate nervous structures and 

 also furnish a convenient medium in which the blood-vessels can ramify. The 

 outer of these is thick and tough, and is termed the dura mater. The intermediate 

 and inner membranes (arachnoid ;nid pia mater) are thin and delicate. These 

 membranes ijresent differences in tlu- regions of the l,>rain and spinal cord res])ec- 

 tively. The membranes of the brain will be first considered, and the dift'erences 

 met with in relation to the spinal cord will be dealt with in the descrij)ti<tn of tiiat 

 region. 



Dissection. — The first step in the examination of tlio meninges is the removal of the brain. 

 The calvaria shituld be removed as fdlows : — The Ixjiie havinir Ix'cn laid bare, a .string should be 

 tied round the skull passing fniui about an inch and a i|uarter alntve the external oeciiiital pm- 

 tuberanee behind, to an inch above the <»r!)ital arches in front. The outer table olthe skull .•should 

 then be .siwn through and the inner table afterwards cracked with the mallet and chisel. A slight 

 wrench will now disengage the calvaria. AfhT noticing the I'acchionian bodies an<l the menin- 

 geal arteries, the student will do well to examine the superior lonLdtmlinal sinus by laying it open. 

 He should next cut throutrh the dura mater in an antero-jtosterior direction on each side of tlie 

 siims, and then make incisions directed transversely outwards from the central jioints (d' the two 

 previous incisions as far as the cut margin of the bone. The four triangular flajx-^ thus mapped 

 out sh(juld be turned downwards, and by gently drawimr one of the hemispheres aside, the lalx 

 cerebri may be seen in situ within the L^reat longitmlinal fis.-ure, and the veins entering the .^ujierior 

 longitudinal sinus may be noted. The falx should then be divided dose to its attaclnnent to 

 the crista galli and thrown backwards. The head should next be inclined liackwards and the frontal 



