CHAPTER V. 

 The Steuctuke oe the Spinal Coed. 



In the introductory chapters the fundamental elements of which the- 

 nervous system is composed were presented; also their arrangement in 

 larger and smaller complexes, and the development of these and the princi- 

 pal divisions of the central system. In the following chapters will be pre- 

 sented the most important facts known regarding the structure of these 

 principal divisions, beginning with the description of the best-known nerv- 

 ous systems: the mammalian and human. 



First to be considered is the spinal cord, the lowest central nervous 

 system, the one which is always present, and which forms the first place of 

 reception and place of origin of nerves. On either side of it lie the spinal 

 ganglia from whose cells the sensory nerves take their origin. They have 

 been found in vertebrates of all classes. The ganglia are constructed of 

 cells whose large bodies send out, in fishes, an axis-cylinder in each direction, 

 the efferent one being the neuraxon, while the afferent one represents a 

 modified dendrite (see Fig. 16, in Chapter II). The same is true in the 

 embryos of other vertebrates; but in the latter the processes come to lie so 

 close together that the first part of their course is represented by a single 

 stalk, only dividing a little way beyond the point of origin. Pig. 37 shows 

 several such cell-types from spinal ganglia. Wherever it has been investi- 

 gated it has been found that one of the processes ran to the periphery as 

 sensory nerve while the other passed to the central organ. All of these 

 central processes taken together are called the dorsal root of the ganglion. 

 The number of dorsal roots is very different in different animals; even 

 among individuals of the same species there may be small variations. That 

 depends upon the length of the animal and the number of the metameres 

 which reach structural maturity. Eanged serially along the sides of the 

 spinal cord these sensory roots enter the cord from the dorsal side. After 

 their entrance they divide into ascending and descending branches, and 

 also give off numerous collateral branches, which send twigs into the gray 

 matter. This ascent and descent occurs in the dorsal segment of the spinal 

 cord, and where many such nerves are present the whole bundle of longi- 

 tudinally ranged nerves is called the posterior tracts. 



Any section of the spinal cord shows that that organ is traversed 

 centrally and longitudinally by a canal — canalis centralis — surrounded by 



(62) 



