THE MEDULLA SPINALIS OR SPINAL CORD 



749 



s origin in the posterior nerve roots. The motor or efferent nerves arise as out- 

 growths of the neuroblasts situated in the basal laminae of the mid- and hind- 

 brain. While, however, the spinal motor nerve roots arise in one series from the 

 basal lamina, the cranial motor nerves are grouped into two sets, according as 

 they spring from the medial or lateral parts of the basal lamina. To the former 

 set belong the oculomotor, trochlear, abducent, and hypoglossal nerves; to the 

 latter, the accessory and the motor fibers of the trigeminal, facial, glossopharyn- 

 geal, vagus nerves (Figs. 659, 660). 



Roof-piate 



Alar lamina 

 Basal lami 



Rhombic lip 



Tractus 

 solitarius 



Vagus nerve 



Hypoglossal nercs 



Floor-plate 









FIG. 660. Transverse section of medulla oblongata of human embryo. (After His.) 



THE MEDULLA SPINALIS OR SPINAL CORD. 



The medulla spinalis or spinal cord forms the elongated, nearly cylindrical, part 

 of the central nervous system which occupies the upper two-thirds of the. vertebral 

 canal. Its average length in the male is about 45 cm., in the female from 42 to 43 

 cm., while its weight amounts to about 30 gms. It extends from the level of the 

 upper border of the atlas to that of the lower border of the first, or upper border 

 of the second, lumbar vertebra. Above, it is continuous with the brain; below, it 

 ends in a conical extremity, the conus medullaris, from the apex of which a delicate 

 filament, the filum terminate, descends as far as the first segment of the coccyx 

 Fig. 661). 



The position of the medulla spinalis varies with the movements of the vertebral 

 column, its lower extremity being drawn slightly upward when the column is 

 flexed. It also varies at different periods of life; up to the third month of fetal 

 life the medulla spinalis is as long as the vertebral canal, but from this stage onward 

 the vertebral column elongates more rapidly than the medulla spinalis, so that by 

 the end of the fifth month the medulla spinalis terminates at the base of the sacrum, 

 and at birth about the third lumbar vertebra. 



The medulla spinalis does not fill the part of the vertebral canal in which it lies; 

 it is ensheathed by three protective membranes, separated from each other by two 

 concentric spaces. The three membranes are named from without inward, the 

 dura mater, the arachnoid, and the pia mater. The dura mater is a strong, fibrous 

 membrane which forms a wide, tubular sheath; this sheath extends below the ter- 

 mination of the medulla spinalis and ends in a pointed cul-de-sac at the level of the 

 lower border of the second sacral vertebra. The dura mater is separated from the 

 wall of the vertebral canal by the epidural cavity, which contains a quantity of loose 

 areolar tissue and a plexus of veins; between the dura mater and the subjacent 

 arachnoid is a capillary interval, the subdural cavity, which contains a small quan- 

 ity of fluid, probably of the nature of lymph. The arachnoid is a thin, transparent 



