214 The Spinal Cord 



space between the arachnoid and pia mater sub-arachnoidean ; in 

 the cranium the fluid is partly in the sub-arachnoid space and 

 partly in the cavity of the brain itself, the communication being through 

 the Sylvian aqueduct. When the fluid escapes from a spina bifida 

 the brain as well as the cord is drained, and, its support being lost, 

 fatal convulsions usually occur. The track of the fluid is first through 

 the foramen of Majendie and then through the aqueduct of Sylvius. 



Inflammation of the membranes (/iefiy) of the cord renders them 

 more thick (iraxvs) t the disease being called pachy-meningitis. The 

 thickening causes pressure on the posterior roots of the spinal nerves 

 and gives rise to peripheral pains (p. 209). This is common in caries 

 and in fracture of the spine. 



On account of the cord ending at the first lumbar vertebra, the 

 lumbar and sacral nerves obviously cannot be given off at the level of 

 their exit from the inter-vertebral foramina. The high cervical nerves 

 have but slight obliquity, but from the dorsal enlargement the direction 

 of the nerves is almost vertical, the sacral nerves descending through 

 the entire length of the lumbar region. 



The situation of the nerves in the canal. The spine of the 

 vertebra prominens corresponds to the origin of the first dorsal nerve 

 from the spinal cord, the spine of the first dorsal to the origin of the 

 third nerve, and the fifth spine to that of the seventh pair. In fact, all 

 through the dorsal region the nerve emerges from the cord at the level 

 of the spine of the vertebra two above. The nerves of the lumbar and 

 sacral plexus arise in the interval between the eleventh dorsal and the 

 first lumbar spines that is from the lumbar enlargement (v. p. 216). 



In the cervical region the nerves take the number of the vertebra 

 above which they pass out, but in the other regions they pass out 

 below the vertebra whose number they bear, the reason being that 

 there are eight cervical nerves, but only seven cervical vertebras, the 

 first nerve emerging above the first vertebra. 



In the brain the grey matter is for the most part on the exterior ; 

 in the cord it occupies a central position, and in transverse section it is 

 seen somewhat in the form of the letter H A slender central canal runs 

 in it, opening above in the floor of the fourth ventricle. It is the ( per- 

 manent remains of the ectodermal canal from which the spinal cord is 

 developed.' Sometimes at birth it is enormously distended with fluid, 

 causing that variety of spina bifida known as syringo-myelocele (<rvpiy, 

 pipe ; /iueXoy, marrow ; KijA?;, tumour). 



The arteries of the cord, which first break up in a delicate network 

 in the pia mater, are derived from the vertebral (anterior and posterior 

 spinal), ascending cervical, intercostal, and lumbar. Of the veins, two 

 are found in the grey commissure, one on either side of the central 

 canal, and others in the anterior and posterior median fissures. They 

 are tributaries of the spinal veins. 



Every spinal nerve arises by two roots, an anterior and posterior, 



