324 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



The development of myelin in the nerve fibers of the cord begins in the fifth month of 

 fetal life and is completed between the fifteenth and twentieth years (Flechsig, Bechterew). 

 Myelin appears first in the root fibers of the spinal nerves and in those of the ventral commissure, 

 next in the ground bundles, and dorsal funiculi. The cerebrospinal (pyramidal) fasciculi are 

 the last in which myelin is developed; they are myelinated during the first and second years. 

 As myelin appears in the various fiber tracts at different periods, this fact has been utilized in 

 tracing the extent and origin of the various fasciculi in the central nervous system. 



The Cervical and Lumbar Enlargements. At the levels of the two nerve 

 plexuses supplying the upper and lower extremities the size of the spinal cord is 



increased. As the fibers to the mus- 

 cles of the extremities arise from nerve 

 cells in the ventral gray column, the 

 number of these cells and the mass of 

 the gray substance is increased; also 

 larger numbers of fibers enter the cord 

 from the integument of the limbs, so 

 that there are larger numbers of cells 

 about which sensory fibers terminate. 

 There is formed consequently at the 

 level of the origin of the nerves of the 

 brachial plexus the cervical enlarge- 

 ment, opposite the origins of the 

 nerves of the lumbo-sacral plexus the 

 lumbar enlargement (Fig. 311). 



Cervical enlargement 



Lumbar enlargement 



FIG. 311. Dissection of the brain and cord of 

 a three months' fetus, showing the cervical and 

 lumbar enlargements (after Kolliker in Marshall). 

 c, cerebellum; h, cerebrum; m, mid-brain. Nat- 

 ural size. 



At the caudal end of the neural 

 tube in an ii cm. fetus an epithelial 

 sac is formed which is adherent to the 



integument. Cranial to the sac the central canal is obliterated and this part of 

 the neural tube forms the filum terminale. The caudal end of the central canal 

 is irregularly expanded and is known as the terminal ventricle. 



The vertebral column during and after the third month grows faster than the 

 spinal cord. As the cord is fixed to the brain it is carried cranially with reference 

 to the vertebrae, and with it shift the roots and ganglia of the spinal necves. In 

 the adult the origin of the coccygeal nerves is opposite the first lumbar vertebra 

 and the nerves course obliquely downward nearly parallel to the spinal cord. As 

 the neural tube is drawn cranially and its caudal tip is attached to the coccyx, its 

 caudal portion is stretched into the slender solid cord known as the filum terminale. 



