850 



THE NERVE SYSTEM 



Dura 



Structure. The pia of the cord is less vascular in structure, but thicker and denser, than 

 the pia of the brain, with which it is continuous. It consists of two layers; an outer, resembling 

 the arachnoid, composed of bundles of connective-tissue fibres, arranged for the most part longi- 

 tudinally; and an inner (intima pia), consisting of stiff circular bundles of the same tissue, 

 which present peculiar angular bends. It is covered on both surfaces by a layer of endothelium. 

 Between the two layers are a number of cleft-like lymphatic spaces which communicate with 

 the subarachnoid cavity, and a number of bloodvessels which are enclosed in perivascular 



sheaths, derived from the inner layer of the pia, 

 into which the lymphatic spaces open. The pia 

 contains the anterior or ventral spinal artery and 

 its branches, the two posterior or dorsal spinal 

 arteries, and numerous veins which pass to the 

 external spinal veins. It is also supplied with 

 nerves, which are derived in part from the sym- 

 pathetic and in part from the cerebrospinal nerves. 

 These nerves supply the walls of the bloodvessels 

 and enter the cord with the vessels. 



Both the arachnoid and the pia may be referred 

 to by the compound name pia-arachnoid, inasmuch 

 as these meningeal components are with difficulty 

 separable from each other. 



Ganglion 

 interval 



Arachnoid 

 Dorsal root 

 Ventral root 



Dura 



z2?j^ Vasa vertebralia 



Plexus venosvs 



FIG. 618. Transverse section of the spinal 

 cord and its membranes. (Gegenbauer.) 



The dentate ligament (liyamentum denti- 

 culatum) (Figs. 596 and 616) is a narrow, 

 fibrous band, situated on each side of the 

 4 spinal cord, throughout its entire length, 



running from the pia to the dura, and separating the ventral from the dorsal 

 roots of the spinal nerves. It has received its name from the serrated appearance 

 which it presents. Its inner border is continuous with the pia at the side of the 

 cord. Its outer border presents a series of triangular, dentated serrations, the 

 points of which are fixed at intervals to the dura. These serrations are twenty- 

 one in number on each side, the first being attached to the dura opposite the 

 margins of the foramen magnum between the vertebral artery and the hypoglossal 

 nerve, and the last near the lower end of the cord. Its use is to support the cord. 



Applied Anatomy. Evidence of value in the diagnosis of meningitis may be obtained by 

 the operation of lumbar puncture, that is, by puncturing the theca of the cord and withdrawing 

 some of the cerebrospinal fluid, and the operation is regarded by some as curative, under the 

 supposition that the draining away of the cerebrospinal fluid relieves the patient by diminish- 

 ing the intercranial pressure. Lumbar puncture may give important diagnostic aid after a 

 head injury by disclosing bloody cerebrospinal fluid. The operation is performed by inserting 

 a trocar of the smallest size below the level of the fourth lumbar vertebra. In an adult the 

 cord terminates at the lower border of the first lumbar vertebra, and in a child opposite the 

 body of the third lumbar vertebra. The canal may be punctured below the fourth vertebra 

 without much risk of injuring its contents. The point of puncture is indicated by laying the 

 child on its side and dropping a perpendicular line from the highest point of the crest of the 

 ilium; this will cross the upper border of the spine of the fourth lumbar vertebra. In a child 

 the puncture is made just below this spine; in adults, one-half an inch to one side of the end 

 of this spine. However the preliminary puncture is made, the needle penetrates the dura 

 in the midline. In entering the needle it should be directed upward and forward in a child; 

 upward, forward, and slightly inward in an adult. 



THE BRAIN OR ENCEPHALON. 



The brain is that greatly modified and enlarged portion of the cerebrospinal 

 axis which, with its membranes, almost completely fills the cavity of the cranium. 

 It is a complex organ in which reside the highest functions consciousness, 

 ideation, judgment, volition, and intellect together with the centres of special 

 sense and for the mechanisms of life (respiration and circulation), and it is the 

 agent of the will. 



