George L. Streeter 19 



The lateral group consists of more or less imiformly large multipolar 

 cells, which in finer histology closely resemble the motor cells of the 

 ventral horns of the higher vertebrates. Their distribution in typical 

 sections is shown in Fig. G. They are seen in every section, but vary in 

 number, being most numerous in the lumbo-sacral region and least 

 numerous in the cervical segments. Corresponding with the number 

 there is some variation in the size; those in the cervical segments average 

 .03 mm. in diameter, while in the lumlio-sacral region there are many 

 cells over .04 mm. This group may be further subdivided into cells 

 having lateral, dorso-lateral, and ventro-lateral positions. A particularly 

 well-defined group of the ventro-lateral cells occurs in the region of the 

 sinus rhomboideus (Fig. 6, segm. XXXYI). 



If we compare this lateral group with the cells of the human cord as 

 classified by Waldeyer'' it is apparent that it corresponds to his median 

 and lateral groups, each of which he subdivides into anterior, middle and 

 posterior subgroups. Tlie cells of the lateral group in segment XXXVI 

 coidd have been separated in a similar manner into a median and a lateral 

 group; the ventro-lateral cells would then Avell correspond to Waldeyer's 

 median group, and the lateral group could l)e further sulidivided into 

 anterior, middle and posterior groups. Such a classification in the ostrich 

 however serves only irregularly and for isolated segments, and therefore 

 this distinction between the cell groiips was not attempted ; but all the 

 large multipolar cells of the ventral horns, the so-called motor cells, were 

 put under the one general class, the lateral group, as described above. 



]\Iost of the cells of the lateral group apparently send their axis-cylin- 

 ders into the ventral nerve-roots. The axis-cylinders of the ventro- 

 lateral cells, however, seem to enter the commissura ventralis. Xo 

 attempt to establish such relations could be made without Golgi prepa- 

 rations, and these unfortunately were not to be had from our material. 



The central group occupies the area of junction of the ventral and 

 dorsal horns, and invades the territory of the horns proper. It con- 

 sists of loosely-scattered cells which vary greatly in size and average a 

 third smaller than the cells of the lateral group. They also stain less 

 intensely and have fewer processes, consequently having less tendency 

 to a multipolar form. 



In the lumbo-sacral enlargement there appear cells among this group 

 which from their size we may speak of as giant cells. They are quad- 

 rilateral or rounded in shape, and vary from .03 to .09 mm. in diameter. 



" Waldeyer, Das Gorillariickenmark, Abhandl. der kgl. preuss. Akad. der 

 Wissensch. zu Berlin, von Jahre 



