420 EMBRYOLOGY. 



At the beginning in Man up to the fourth month of embryonic 

 development the spinal cord occupies the entire length of the body. 

 Therefore, at the time when the axial skeleton is divided up into 

 separate vertebral regions, it reaches from the first cervical down to 

 the last coccygeal vertebra. The end of the spinal cord, however, 

 does not even begin to develop ganglionic cells and nerve-fibres, but 

 remains throughout life as a small epithelial tube. It is united to 

 the larger anterior portion, which has developed nerve-fibres and 

 ganglionic cells, by means of a conically tapering region, which is 

 spoken of in descriptive anatomy as the conus medullaris. 



As long as the spinal cord keeps pace with the vertebral column 

 in its growth, the pairs of nerves arising from it, in leaving the 

 vertebral canal, pass out at right angles directly to the intervertebral 

 foramina. In Man, beginning with the fourth month, this arrange- 

 ment is changed ; from that time forward the growth of the spinal 

 cord does not equal that of the spinal column, and therefore the cord 

 can no longer occupy the entire length of the vertebral canal. Since 

 it is attached above to the medulla oblongata, and this together with 

 the brain is firmly held in the cranial capsule, it must assume a higher 

 and higher position in the vertebral canal. In the sixth month the 

 conus medullaris is found in the upper end of the sacral canal, at birth 

 in the region of the third lumbar vertebra, and some years later at 

 the lower edge of the first lumbar vertebra, where it terminates 

 even in the adult. 



In the ascent (ascensus medullae spinalis) the lower end of the 

 spinal cord, the small epithelial tube which is attached to the coccyx, 

 is drawn out into a long, fine filament, which persists even in the 

 adult as the filum terminate internum and externum. At first it 

 presents a small cavity, which is lined by ciliated cylindrical cells, 

 and which forms a continuation of the central canal of the spinal 

 cord. Further downward it is continued in the form of a cord of 

 connective tissue as far as the coccyx. 



A second consequence of the ascent of the spinal cord is a change 

 in the course of the roots of the peripheral nerve-stems. Since, together 

 with the spinal cord, their points of origin come to lie in the spinal 

 canal relatively nearer and nearer the head, and since the places where 

 they pass through the intervertebral foramina do not change, they 

 are compelled to pass from a transverse to a more and more oblique 

 course. The obliquity, moreover, is greater the farther down the 

 nerve leaves the vertebral canal. In the neck-region their direction 

 is still transverse, in the thoracic region it begins to be more and 



