326 



THE MORPHOGENESIS OF THE CENTRAL- NERVOUS SYSTEM 



Cerebrum 

 Mesencephalon 



Cerebellum 



Cervical 

 enlargement 



Lumbar 



enlargement 



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. 



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 number 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 like- 

 wise present more 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 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 of a 

 four months' fetus, an epithelial sac is formed 

 which is adherent to the integument. Cranial to 

 the sac, the central canal is obliterated, this part 

 of the neural tube forming the filum terminate. 

 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 vertebras and the asso- 

 ciated roots and ganglia of the spinal nerves shift 

 caudally along the cord. The origin of the 

 coccygeal nerves in the adult is opposite the first 

 lumbar vertebra and the nerves course obliquely 

 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, solid cord known as the filum terminate. 

 The obliquely coursing spinal nerves, with the filum terminale, constitute 

 the cauda equina. 



THE BRAIN 



We have seen that in embryos of 2 to 2.5 mm. the neural tube is 

 nearly straight, but that its cranial end is enlarged to form the anlage 

 of the brain (Fig. 324). The appearance of two constrictions in the wall 

 of the anlage subdivides it into the three primary brain vesicles the fore- 

 brain, or prosencephalon, mid-brain, or mesencephalon, and hind-brain, 

 or rhombencephalon. 



FIG. 329 Dissection of 

 the brain and cord of a three 

 months' fetus, showing the 

 cervical and lumbar enlarge- 

 ments (after Kolliker in 

 Marshall). Natural size. 



