THE SPINAL CORD AND SIMNAL NKKVKS. 527 



Other investigators hold that in the further elongation of the 

 axis cylinder, after its first appearance, the cells in the neigh- 

 bourhood are actively concerned, the nerve fibre being formed 

 either by the linear fusion of originally independent cells, or as 

 a process of secretion by the surrounding cells. The balance of 

 evidence at present appears to be decidedly in favour of the 

 first-mentioned view, i.e. that a nerve fibre is to be regarded 

 throughout its whole length as a process of a single nerve cell. 



The blood-vessels of the spinal cord do not appear until the 

 beginning of the fifth week ; they are carried into the cord by 

 connective tissue, which grows into its substance from without. 



The spinal cord steadily increases in diameter, mainly through 

 the formation of the longitudinal bands of white matter, i.e. 

 of nerve fibres, on its outer surface. The median fissures of the 

 cord are formed in the same way as in other Vertebrates, the 

 ventral fissure being a chink left between the ventral columns 

 of the cord ; while the dorsal fissure is of entirely different 

 origin, and is due to the absorption of the substance of the cord 

 along the dorsal surface in the median plane. 



The seat of most active nerve growth in the early stages is 

 the neck, the cervical nerves being, both relatively and abso- 

 lutely, larger than the hinder ones during the early stages. 



The cervical and brachial plexuses commence to form about 

 the twenty-seventh day ; the lumbo-sacral plexus rather later, 

 about the thirtieth day (Fig. 227). The phrenic nerve appears 

 about the thirtieth day as a branch of the fourth cervical 

 nerve. 



The cervical and lumbar enlargements of the spinal cord are 

 present in the second month, and are well marked by the end of 

 the third month (Fig. 218). 



The spinal cord originally extends to the last caudal ver- 

 tebra ; and up to the end of the third month the growth of the 

 spinal cord keeps pace with that of the vertebral column. From 

 the fourth month onwards the vertebral column grows more 

 rapidly. By the sixth month the spinal cord only extends to 

 the sacral Vertebrae ; at birth it stops at the third lumbar 

 vertebra, while in the adult its lower end is opposite the lower 

 border of the first lumbar vertebra. This shortening of the 

 spinal cord relatively to the vertebral column is the cause of the 

 obliquity of the roots of the hinder spinal nerves, which have to 



