882 THE NERVOUS SYSTEM 



plexus mainly by the radicular veins, which pass along the roots of the last four cranial nerves. 

 Those which accompany the hypoglos.sal nerves have already been referred to. 'J he others end 

 in the terminal parts of the transverse sinuses, the inferior putrosal sinuses, or the lower part of 

 the occipital sinuses. 



The nerve supply of the blood-vessels of the brain consists of a ]ri vascular plexus of sympa- 

 thetic nerve-fibres upon the walls of the vessels and medullated fibres which accompany the 

 vessels and apparently terminate, for the most part, in the connective tissue about them. The 

 former are thought to be vaso-motor in function; the latter probably sensory fibres of the 

 cerebro-spinal type. Nerves have been described only for the larger vessels. 



THE MENINGES 



Three membranes, collectively called the meninges, envelope the entire central 

 nervous system, separate it from the walls of the bony cavities in which it lies, and 

 aid in its protection and support. They consist of feltworks in which white fibrous 

 connective tissue predominates, and through them pass the blood-vessels which supply 

 the central nerve-axis and the nerves by which the axis is connected with the periphery. 

 Though there are definite spaces or cavities between them, the membranes are not 

 wholly separated from each other, and they are both continuous with and contribute 

 to the walls of the blood-vessels and the sheaths (epineurium) of the nerves passing 

 through them. Beginning with the outermost, they are (1) the dura mater, the 

 thickest, most dense, and resistant of the membranes; (2) the arachnoid, the much 

 less dense and more serous middle membrane; and (3) the pia mater, a thin, com- 

 pact membrane, closely adapted to the surface of the central system, into which it 

 sends numerous connective-tissue processes. It is highly vascular in that it contains 

 the rich superficial plexuses of blood-vessels from which the intrinsic blood supply of 

 the central system is derived. The space between the dura mater and the arachnoid 

 is known as the sub-dural cavity, and that between the arachnoid and the pia mater 

 is the sub-arachnoid cavity. 



* 



THE DURA MATER 



In the fresh condition the dura .mater appears as a bluish-white, exceed- 

 ingly resistant membrane, forming the outermost envelope of the entire central 

 nervous system. Its external surface or that next to the bony wall is rough, 

 while its internal surface appears smooth, due to the fact that the subdural cavity 

 partakes of the nature and has the lining of a lymph-space. The cranial dura mater 

 consists of two distinct, closely associated layers, the outermost of which serves as the 

 internal periosteum of the cranial bones. The spinal dura mater is described as con- 

 sisting of but one layer. The internal periosteum of the spinal canal, though con- 

 tinuous at the foramen magnum with the outer layer of the cranial dura mater, is not 

 considered a part of the spinal dura mater, from the fact that it is so widely separated 

 from the layer actually investing the spinal cord. Thus, since the cranial and spinal 

 portions of the dura mater differ, they are described separately. 



The spinal dura mater is a fibrous tube with funnel-shaped termination which 

 encloses and forms the outermost support of the spinal cord. It consists of but 

 one layer, and this corresponds to the inner layer of the cranial dura mater. It begins 

 at the foramen magnum and terminates in the spinal canal at about the level of the 

 third piece of the os sacrum. It is firmly attached to the periosteum of the surround- 

 ing bones only in certain localities: 



(1) The upper end of the tube blends intimately with the periosteum of the mar- 

 gin of the foramen magnum, and thus in this locality it becomes continuous with the 

 outer layer of the cranial dura mater. Also in this locality it is attached firmly, 

 though less intimately, to the periosteum of the posterior surfaces of the second and 

 third cervical vertebra*. This locality may be considered the upper fixation-point of 

 the spinal dura mater. (2) It extends laterally and contributes to the connective tissue 

 investments of each pair of spinal nerves, and as such it passes into the intervertebral 

 foramina and becomes loosely connected with the periosteum lining each. (3) Along 

 its ventral aspect the spinal dura mater is attached by numerous processes to the 

 posterior longitudinal ligament of the vertebral canal. These attachments are more 

 or less delicate, loose, and irregular, and are easily torn or cut in removing the speci- 

 men. They are stronger and more numerous in the cervical and lumbar regions than 



