326 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



The development of myelin in the nerve fibers of the cord begins late in the fourth 

 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 myehn is developed; they are myelinated during 

 the first and second years. As myelin appears in the various fiber tracts at different periods, 

 this condition has been utilized in tracing the extent and origin of the various fasciculi in 

 the central nervous system. 



Cerebrum 



Mesencephalon 



Cerebellum 



Cervical 

 enlargement 



Lumbar 

 enlargement 



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

 plexuses, supplying the upper and lower extremities, the spinal cord enlarges. 



As the fibers to the muscles of the extremities arise 

 from nerve cells in the ventral gray column, the num- 

 ber of these cells and the mass of the gray substance 

 is increased; since larger numbers of fibers from the 

 integument of the limbs also enter the cord at this 

 level, there are likewise present more cells about 

 which sensory fibers terminate. There is formed con- 

 sequently at the level of the origin of the nerves of 

 the brachial plexus the cervical enlargement, opposite 

 the origins of the nerves of the lumbo-sacral plexus 

 the lumbar enlargement (Fig. 329). 



At the caudal end of the neural tube in a 110 ram. 

 (C R) fetus an epitheKal sac is formed which is ad- 

 herent to the integument. Cranial to the sac the 

 central canal is obHterated, this part of the neural 

 tube forming the filum terminale. The caudal end 

 of the central canal is irregularly expanded and is 

 known as the terminal ventricle. 

 After the third month the vertebral column grows faster than the spinal cord. 

 As the cord is fixed to the brain the vertebrae and the associated roots and gangha 

 of the spinal nerves shift caudally along the cord. In the adult the origin of the 

 coccygeal nerves is opposite the first lumbar vertebra and the nerves course ob- 

 liquely downward nearly parallel to the spinal cord. As the tip of the neural 

 tube is attached to the coccyx, its caudal portion becomes stretched into the 

 slender, soKd cord known as the filum terminale. The obliquely coursing spinal 

 nerves, with the filum terminale, constitute the cauda equina. 



Fig. 329.— Dissection of 

 the brain and cord of a three 

 months' fetus, showing the cer- 

 vical and lumbar enlargements 

 (after Kolliker in Marshall). 

 Natural size. 



