836 COMPARATIVE ANATOMY 



by a cytoplasmic outgrowth from the neuroblast itself. Such processes 

 may be several feet in length or very short. Some of the little fibres 

 produced in this way may extend out from the cord as individual nerves, 

 while others run longitudinally within the cord. Others run on the out- 

 side of the cord longitudinally. Those that run along the outer portion 

 of the cord are often called the marginal layer because they form a sort 

 of envelope of fibers for trie neural cord itself. These fibers are medu- 

 lated or covered with a white substance and this white envelope is called 

 the white matter of the cord. That portion lying further toward the 

 lumen. and composed largely of cell bodies is what constitutes the gray 

 matter. 



In cross-sections of the spinal cord of*a higher vertebrate there will 

 be seen a portion looking something like the capital letter "H" with a 

 central canal in the middle of the cross-bar. The entire substance which 

 looks like the letter "H" is the gray matter of the cord. The dorsal up- 

 right bars of the "H" form the posterior columns, while the ventral up- 

 rights form the anterior columns of the cord. Immediately lateral to the 

 crossbar on each side of the cord there is another column known as the 

 lateral column. This latter differs to a considerable extent not only in its 

 relation but also in its function from the dorsal and ventral column. 

 This H-shaped gray matter really divides the white matter into three 

 longitudinal tracts called funiculi, formerly also called columns. They 

 are known as the dorsal, ventral, and lateral funiculi. 



It will be remembered th'at the white matter is composed of longi- 

 tudinal fibres. It is these longitudinal fibres which make up the various 

 funiculi which connect the different parts of the central nervous system 

 with each other. It is important to remember that these fibres are not 

 all alike, but that those in the dorsal funiculus carry impulses toward the 

 brain and' are therefore called ascending tracts; while the ventral funi- 

 culus is known as the descending tract in that it carries fibres from the 

 brain downward. The lateral funiculi have fibres of both kinds and carry 

 impulses in both directions. 



The fibres in each of the funiculi are again grouped into smaller 

 oundles or fasciculi, each with its own name. Some of the fibres coming 

 from the brain are distributed at different levels along the cord, while 

 others, going to the brain, are added to the funiculi at different places. 

 The size of the funiculi thus decreases with the distance from the 

 brain. Some of the bundles may disappear in the more distal parts of 

 the cord. 



The spinal cord is approximately cylindrical in the higher animal 

 forms, but in the lower it is flattened dorsoventrally, the flattening being 

 greatest in the cyclostomes. In the lower groups there is also a differ- 

 ence in the shape of the gray matter, the H shape being less distinct. 

 i; 'The cord tapers pretty regularly in fishes, from the brain to, the 



