THE CEEEBBO-S PINAL AXIS. G61 



resolves itself into two layers : an external, applied to the inner face of the 

 dura mater ; and an internal, spread, through the medium of the pia mater, 

 over the cerebro-spinal axis, from which it is again separated in a great 

 number of points by a particular fluid, the svharmhnoid. 



The pia mater, or internal meninge, is the proper envelope of the central 

 nervous stalk ; it is cellulo-vascular, closely adherent to the external surface 

 of the cord, united to the visceral layer of the arachnoid by more or less 

 dense connective tissue, between the meshes of which is deposited the 

 subarachnoid 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 

 sheath containing them to a splanchnic cavity, whose serous membrane, 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 meninge. 



This collective view of the envelopes belonging to the nervous centres 

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

 and cranial portions wil be successively considered, after glancing at them 

 in a general manner. 



1. Tlie Dura Mater. 



This membrane is the most external and the strongest of the cerebro- 

 spinal envelopes, and covers the walls of the cerebro-spinal cavity, whose 

 foi-m it exactly repeats. It is, therefore, a second protective sheath, which 

 is dilated at its anterior extremity into an ovoid cavity that lodges the 

 encephalon, and terminates in a prolonged point in the coccygeal vertebrte. 



It offers two faces : an external, in contact with the walls of the bony 

 case ; and an internal, adhering in the most intimate manner to the external 

 layer of the arachnoid. 



In several points of its extent it is traversed by the nerves which escape 

 from the cerebro-spinal axis, and by the vessels destined to this portion of 

 the nervous system. 



Steucttjee. — The dura mater possesses the texture of all white fabrous 

 membranes. 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 Uood-vessels ; the arteries are derived, for the 

 spinal portion, from the vertebral, the 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 cramal 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 ophthalmic branch of WiUis. 

 The existence of lymphatic vessels has not yet been demonstrated. 



Spinal Duea Mater (Thbca Veetkbbalis).— This is a very elongated 

 sheath, continuous at the occipital foramen with the encephalic dura mater and 

 terminated behind by an attenuated point lodged m the narrow channel which, 

 in the middle coccygeal vertebra, represents a trace of the spmal canal. 

 As it is in shape exactly like the latter, its largest diameter is at the atlas, 

 and at the brachial and lumbosacral enlargements of the spinal cord. Its 

 capacity depends greatly on the volume of the latter, and m some of its 

 parts it can allow the accumulation of the cerebro-spmal fluid; this 

 45 



