THE NERVOUS SYSTEM. 



Nervous System is composed : 1. Of a series of large centres of nerve- 

 JL matter, called, collectively, the cerebro-spinal centres or cerebro-spinal axis. 

 2. Of smaller centres, termed ganglia. 3. Of nerves connected either with the 

 cerebro-spinal axis or the ganglia. And 4. Of certain modifications of the periph- 

 eral terminations of the nerves forming the organs of the external senses. 



The Cerebro-spinal axis consists of the brain or encephalon and the spinal 

 cord, which are contained within the skull and spinal canal. The brain and its 

 membranes will be first considered, and then the spinal cord and its coverings. 



THE MEMBRANES OF THE BRAIN. 



Dissection. To examine the brain with its membranes, the skull-cap must be removed. 

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

 an inch above the margin of the orbit, and extending, behind, to a little above the level with 

 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 skull-cap, when the 

 dura mater will be exposed. The adhesion between the bone arid the dura mater is very 

 intimate, and much more so in the young subject than in the adult. 



The membranes of the brain are : the dura mater, arachnoid membrane, and 

 pia mater. 



The Dura Mater. 



The Dura Mater is a thick and dense inelastic fibrous membrane which lines 

 the interior of the skull. Its outer surface is rough and fibrillated, and adheres 

 closely to the inner surface of the bones, forming their internal periosteum, this 

 adhesion being most marked opposite the sutures and at the base of the skull. 

 Its inner surface is smooth and lined by a layer of endothelium. It sends four 

 processes inward, into the cavity of the skull, for the support and protection of 

 the different parts of the brain, and is prolonged to the outer surface of the skull 

 through the various foramina which exist at the base, and thus becomes contin- 

 uous with the pericranium ; its fibrous layer forms sheaths for the nerves which 

 pass through these apertures. At .the base of the skull it sends a fibrous prolon- 

 gation into the foramen caecum ; it sends a series of tubular prolongations round 

 the filaments of the olfactory nerves as they pass through the cribriform plate, 

 and also round the nasal nerve as it passes through the' nasal slit ; a prolongation 

 is also continued through the sphenoidal fissure into the orbit, and another is con- 

 tinued into the same cavity through the optic foramen, forming a sheath for the 

 optic nerve, which is continued as far as the eyeball. In the posterior fossa it 

 sends a process into the internal auditory meatus, ensheathing the facial and 

 auditory nerves ; another through the jugular foramen, forming a sheath for the 

 structures which pass through this opening; and a third through the anterior 

 condyloid foramen. Around the margin of the foramen magnum it is closely 

 adherent to the bone, and is continuous with the dura mater lining the spinal 

 canal. In certain situations, as already mentioned (page 594), the fibrous layers 

 of this membrane separate, to form sinuses for the passage of venous blood. 

 Upon the outer surface of the dura mater, in the situation of the longitudinal 

 sinus, may be seen numerous small whitish bodies, the glandule? Pacchioni. 



Structure. The dura mater consists of white fibrous tissue with connective- 

 tissue cells and elastic fibres arranged in flattened laminae which are imperfectly 

 separated by lacunar spaces and blood-vessels into two layers, endosteal and 

 meningeal. * The endosteal layer is the internal periosteum for the cranial bones, 



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