PRACTICAL CONSIDERATIONS: SPINAL CORD. 1051 



Based on the observations of Flechsig, His, Bechterew, and others, the time of the 

 appearance and of the development of the medullary coat of some of the fibres within the 

 spinal cord may be given. 



Fibres of Appear Myelinate 



Anterior root about 4th week during 5th month 



Burdach's tract during 4th week end of 6th month 



Coil's tract about gth week beginning of 7th month 



Pyramidal tracts end of 5th month 9th month to 2nd^year 



Direct cerebellar tract beginning of 3rd month about 6th montli 



Gowers' tract during 4th month during 6th month 



The presence of the sinus terminalis (page 1030) in the cord at birth depends partly upon 

 the persistence of the lumen of the central canal at the lower end of the conus medullaris and 

 partly upon a proliferation of the wall-cells of the subjacent segment, followed by secondary- 

 dilatation shortly before birth. 



During the early weeks of development, the neural tube extends to the lowermost limits of 

 the series of somites ; but after differentiation of the root-fibres begins, the segment of the cord 

 below the level of origin of the first coccygeal nerves is marked by feeble proliferation, the 

 effects of which are soon manifest in the rudimentary condition of the caudal end of the cord. 

 With the subsequent development of the other regions, this histological contrast becomes more 

 evident, to which is soon added the conspicuous attenuation caused by the attachment of the 

 lower end of the cord to the caudal pole of the spine, which elongates with greater rapidity 

 than the contained nervous cylinder. In this manner the lowest segment of the cord, with its 

 mesoblastic envelope, is converted into the delicate thread-like filum terminate, within whose 

 upper half are found the remains of the rudimentary nervous tissue. 



PRACTICAL CONSIDERATIONS : SPINAL CORD. 



Congenital Errors in Development. The spinal cord may be absent (^amyelia), 

 or it may be defective in a certain portion (atelo myelid). In such conditions, however, 

 the patient cannot live. The cord may be double from bifurcation {diplomyelia). 



A spina bifida is a congenital condition due to a deficiency in the vertebrae, 

 almost always of the laminae and spinous processes. There is usually a protrusion 

 of the contents of the spinal canal, although in some cases there is no protrusion, and 

 in others the vertebral canal, or even the central canal of the cord may be open to 

 the surface. Three varieties of tumors are described according to their contents. If 

 the meninges only protrude from the canal in the form of a sac containing cerebro- 

 spinal fluid, it is called a meningocele ; if the sac contains a portion of the cord also 

 it is called a meningo-myelocele. In the third variety, syringo-myelocele, the cavity 

 of the tumor is found to consist either of the dilated canal of the cord, so that the 

 thinned-out substance of the cord is in the wall of the sac, or of a cavity in the cord 

 tissue itself. This is the least common of the three forms. 



In the meningo-myelocele, which is the most common form, the cord becomes 

 flattened out and attached to the posterior wall of the sac, but still has its central 

 canal intact. The spinal nerves cross the sac to their corresponding intervertebral 

 foramina. In this and in the syringo-myelocele there is frequently some degree of 

 paralysis in the parts below from disturbance of the cord at the seat of the tumor. 

 The most common seat of the defect is in the lumbo-sacral region. It is rare in 

 other parts of the spine. Therefore, the bowels, bladder, and lower extremities are 

 the parts most frequently affected. If the lesion is confined to the lower part of the 

 sacral region, the extremities usually escape. Paralytic talipes is comparatively 

 common. 



There is no sharp line of demarcation between the medulla oblongata and the 

 cord. The beginning of the latter is variously given as at the origin of the first 

 cervical nerve, the lower margin of the foramen magnum, or the decussation of the 

 pyramids, the last being the more generally accepted. 



Since in the adult, the spinal cord ends below usually at the level of the disc 

 between the first and second lumbar vertebrae, injuries of the spine below the second 

 lumbar vertebra do not involve the cord. The membranes of the cord, however, 

 containing cerebro-spinal fluid extend as far as the second or third sacral vertebra, 

 so that at this level injuries with infection may cause fatal meningitis. 



