846 THE NERVE SYSTEM 



(a) Axones arising in the cerebral cortex, the gray ganglionic masses in the 

 mid-brain, pons, and cerebellum, and descending to their terminations in different 

 levels of the cord. lJV\jp.) 



(6) Axones which, conversely, arise in the gray substance of the cord (tract cell 

 axones), to terminate in the higher brain centres. tff\tt&Vvt%^ 



(c) Axones which coordinate different levels of the cord with each other (associa- 

 tion, cell axones). 



(d) Axones which, arising from the spinal ganglion cells of the dorsal nerve 

 roots, enter the cord and ascend in the dorsal columns. 



Dissection. To dissect the cord and its membranes it will be necessary to lay open the 

 whole length of the vertebral canal. For this purpose the muscles must be separated from the 

 vertebral grooves, so as to expose the spinous processes and laminse of the vertebrae; and the 

 latter must be sawed through on each side, close to the roots of the transverse processes, from 

 the third or fourth cervical vertebra above to the sacrum below. The vertebral arches having 

 been displaced by means of a chisel and the separate fragments removed, the dura will be ex- 

 posed, covered by a plexus of veins and a quantity of loose areolar tissue, often infiltrated with 

 serous fluid. The arches of the upper vertebra? are best divided by means of a strong pair of 

 cutting bone-forceps or by a rachitome. 



Applied Anatomy. Contusion of the spine may cause an effusion of blood or traumatic 

 zonal inflammation of the spinal cord with paralysis which may disappear without surgical 

 intervention. Concussion ("railway spine") is followed by temporary or rarely permanent 

 impairment of function. Punctured wounds are usually caused by the knife, and are most com- 

 monly inflicted upon the cervical and upper thoracic divisions and more often involving one- 

 half of the cord. The course is generally an aseptic one; operation is indicated when infection 

 of the cord ensues or when there is a compression from hemorrhage. This may be relieved by 

 lumbar puncture (p. 68) with a large needle. Hemorrhage into the substance of the cord 

 (hematomyelia) usually occurs in the 4-6 cervical segments. If the hemorrhage is confined to 

 the gray substance, there is wasting of muscle and anesthesia of the pectoral limb; the reflexes 

 below the lesion are abolished. If the white substance is involved, paraplegia below the level 

 of the lesion ensue,s. The cord may be injured by fracture or dislocation of vertebrae (p. 69). 

 Gunshot wounds are usually of serious nature. Operation is indicated by symptoms of com- 

 pression by the bullet or by fragments of bone. If the compression is due to accumulated blood, 

 lumbar puncture may be resorted to. 



The cord usually shares in the congenital malformation known as spina bifida, resulting 

 from failure of coalescence of the margins or lateral ridges of the neural plate. The defect may 

 be complete (rachischisis totalis) or confined to a few segments of the cord. Tumors of many 

 varieties, originating in the vertebra, meninges, or even in the substance of the cord itself (glioma, 

 tuberculoma, etc.), give rise to many sensor and motor disturbances which afford a clue to the 

 localization of such growths as indicated on page 831. Inflammation of the spinal cord (myelitis) 

 may follow any of the acute specific fevers, occasioning more or less complete paralysis and 

 anesthesia. In infantile spinal paralysis (acute anterior poliomyelitis) due to a bacterial infec- 

 tion which results in the destruction of ventral horn (motor) cells and consequent permanent 

 paralysis and nutritional disturbances of the limbs whose centres are thus affected. Further, 

 the muscles which normally antagonize the affected groups of muscles tend to assume a state 

 of spastic contraction. The deformity thus produced may be relieved by tenotomy, transplan- 

 tation of tendons, or even amputation. Syrinr/omyelia is a chronic condition in which an 

 abnormal proliferation of the spinal neuroglia takes place, generally near the central canal and 

 in the cervical enlargement, and later this mass becomes absorbed, leaving an irregular cavity 

 in its place. This gives rise to a number of interesting symptoms, such as analgesia (or insen- 

 sitiveness to pain), inability to distinguish between heat and cold, progressive atrophy of the 

 muscles of the hands and arms, trophic changes in the bones and joints, and painless whitlows. 



THE MEMBRANES OF THE CORD. 



The membranes which envelop the spinal cord are three in number. The most 

 external is the dura, a strong fibrous membrane which forms a loose sheath around 

 the cord. The most internal is the pia, a cellulovascular membrane which closely 

 invests the entire surface of the cord. Between the two is the arachnoid, an 

 avascular membrane which envelops the cord and is connected to the pia by 

 slender filaments of connective tissue. 



