THE CEREBROSPINAL AXIS. 749 



nervous mass ; it is celliilo-vascnlar, closely adherent to the external surface of 

 the mass, attached to the visceral layer of the arachnoid by more or less dense 

 connective tissue, between the meshes of which is the sub-arachnoid fluid. 



This arrangement of the cerebro-spinal envelopes permits the cerebro-spinal 

 axis to be assimilated, to a certain extent, to a viscus, and the bony case contain- 

 ing them to a splanchnic cavity, the serous membrane of which — the arachnoid 

 — is covered outside its parietal layer by a fibrous expansion — the dura mater, and 

 within its visceral layer by a cellulo-vascular tunic— the pia mater, or internal 

 men in (/e. 



This collective view of the envelopes belonging to the nerve-centres will 

 now be followed by a special description of each, in which their spinal and cranial 

 portions will be successively considered, after glancing at them in a general 

 manner. 



1. The Dura Mater. 



This is the most external and the strongest of the cerebro-spinal envelopes, 

 and lines the walls of the cerebro-spinal cavity, exactly repeating its shape. It 

 is, therefore, a second protective covering, which is dilated at its anterior 

 extremity into an ovoid cavity that lodges the brain, and terminates in a pro- 

 longed point at the coccygeal vertebne. 



It has two faces — an external, in contact with the walls of the cranium ; and 

 an internal, adhering in the most intimate manner to the external layer of the 

 arachnoid. 



In several parts of its extent it is traversed by the nerves that proceed from 

 the cerebro-spinal axis, and by the vessels for this portion of the nervous 

 system. 



Structure, — The dura mater possesses the texture of all white fibrous mem- 

 branes. It is composed of parallel longitudinal fasciculi of connective tissue, 

 mixed with some fine elastic fibres. Bourgelat thought they formed two distinct 

 layers — an external and internal ; but nowhere is it possible to demonstrate this. 

 It receives bloott-vestsels ; the arteries are derived — for the spinal portion, from 

 the vertebral, intercostals, lumbar, and lateral sacrals ; for the cranial portion, 

 meningeal ramuscules, such as the ethmoidal branch of the nasal, the spheno- 

 spinous, and tympanic, mastoideal, and cerebro-spinal arteries. Nerves have been 

 seen passing to its cranial portion ; these have been divided into anterior, middle, 

 and posterior. The first are furnished by the ethmoidal filament of the nasal 

 nerve ; the second from the Gasserian ganglion ; and the third, by the ophthal- 

 mic branch of Willis. The nerves of the spinal portion are very fine and have 

 no myeline ; at first they accompany the vessels, then leave them to be dis- 

 tributed in the middle part of the membrane. The existence of lymphatic 

 vessels has not yet been clearly demonstrated. 



Spinal Dura Mater (Theca Vertebralis, Dura Mater Spinalis). — 

 This is a very elongated sheath, continuous at the occipital foramen with the 

 cranial dura mater, and terminated behind by an attenuated point lodged in the 

 narrow channel which, in the middle coccygeal vertebrie, represents a trace of 

 the spinal canal. As it is in shape exactly like the latter, its largest diameter is 

 at the atlas, and at the brachial and lumbo-sacral enlargements of the spinal 

 cord. Its capacity depends greatly on the volume of the latter, and in some of 

 its parts it can allow accumulation of the cerebro-spinal fluid ; this is impos- 

 sible, however, for nearly the whole extent of the cranial region. 

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