THE ORGANS OF THE OUTER GERM-LAYER. 421 



more oblique, and finally, in the lumbar region, and still more so in 

 the sacral, it is more sharply downward. On this account the nerve- 

 stems arising from the last part of the spinal cord come to lie for a 

 considerable distance in the vertebral canal before they reach the 

 sacral foramina serving for their exit ; they therefore surround the 

 conus medullaris and filum terminale, forming the structure known 

 as the horse-tail or cauda equina. 



Finally the spinal cord undergoes some changes in its form also. 

 Even in the third and fourth months there appear differences of calibre 

 in different regions. The places in the cervical and lumbar regions 

 of the spinal cord at which the peripheral nerves depart to the anterior 

 and posterior extremities, grow vigorously by the abundant formation 

 of ganglionic cells ; they become considerably thicker than the adjoin- 

 ing portions of the cord, on account of which they are distinguished 

 as cervical and lumbar enlargements (intumescentia cervicalis et 

 lumbalis). 



(b) The Development of the Brain. 



By the study of embryology knowledge of the anatomy of the 

 brain has been greatly promoted. Justly, therefore, in all recent 

 text books of human anatomy, the embryonic condition serves as 

 the starting-point in the description of the intricate structure of the 

 brain, the aim being to derive the complicated ultimate conditions 

 from the more simple embryonic ones, and to explain them by means 

 of the latter. 



The initial form of the brain as well as of the spinal cord is a simple 

 tube. At an early period, even before it is everywhere closed, it 

 becomes metameric, on account of its growth being greater in some 

 regions than in others. By means of two constrictions of its lateral 

 walls it is divided into the three primary brain-vesicles (fig. 87 hb l , hb 2 , 

 hb 3 ), which remain united with one another by means of wide openings, 

 and are designated as the fore-, mid-, and hind-brain. The posterior 

 of these divisions is the longest, gradually tapering and becoming 

 continuous with the tubular spinal cord. 



The first stage is quickly followed by a second, and that by a third, 

 since the primary brain-vesicles soon separate into four, and finally 

 five divisions. 



During the second stage (fig. 234) the lateral walls of the primary 

 fore-brain (pvh) begin to grow outward more vigorously and to 

 evaginate to form the two optic vesicles (au). At the same time the 



