938 ON THE DEVELOPMENT OE THE SPINAL COED IN MAN, MAMMALIA, AND BIRDS. 
Fig. 41. Section through the lowest part of the dorsal region, or upper end of the 
lumbar enlargement. Here the posterior vesicular column [g) is larger than 
in any other part of the cord, and consists chiefly of large, oval and stellate 
cells. Here also we first distinctly see the tractus intermedio-latei'olis (^), a 
tract of smaller cells between the anterior and posterior cornua, and project- 
ing in a conical form from the border of the grey substance into the lateral 
white column. 
Fig. 42. Another section, through the middle of the dorsal region. The posterior vesi- 
cular column diminishes in size. 
Fig. 43. A section through the ujpjper part of the dorsal region ; the tractus intermedio- 
lateralis [f) is very prominent. 
Fig. 44. Another, through the middle of the cervical enlargement : — e\ the posterior 
lateral Assure, through which the outer fibres of the posterior roots {d) are 
seen to reach the dark spots or longitudinal bundles on the outer side of 
the cervix cornu, with which they become continuous. These longitudinal 
bundles are more numerous here than in any other region of the cord. On the 
inner side of the cornu, other fibres of the roots are seen sweeping both ai'ound 
and within the posterior vesicular column [g). This column is here again 
large, but consists chiefly of a multitude of small nerve-cells. The groups of 
nerve-cells in the anterior cornu have again increased in size. These groups, 
and indeed the whole of the anterior cornu, as well as the outer part of the 
cervix of the posterior cornu, are supplied, as represented, on the left side, by 
a beautiful branch of a large blood-vessel {v), which enters through the anterior 
median fissure (m), and bifurcates right and left, at its bottom, through the 
anterior commissure. The parts behind the canal are supplied by other 
vessels running transversely and derived in part from a larger longitudinal 
vessel on each side of the canal, and of which the cut ends are seen in the figure. 
In the lateral white column is a somewhat oval space [g') occupying nearly 
the whole of its area, and of a lighter and greyer colour than the rest. This is 
apparently due to a greater abundance of blood-vessels and pia mater. It is 
limited chiefly to the cervical and dorsal region. The wedge-shaped column 
{])') on each side of the posterior median fissure, and forming part of the 
posterior white column, is very strongly marked in this figure. Its tapering 
end is gradually lost in the deep part of the column on its outer side. At 
this period they are limited almost entirely to the cervical enlargement; at 
an earlier period they may be traced lower down (see figs. 4, 5, 6, & 7). 
Fig. 45. Part of the chorda dorsalis of the chick at the end of the ninth day of incuba- 
tion, with some of the surrounding cartilage-cells: — a, chorda dorsalis, con- 
sisting of a nucleated network of fibres, having precisely the appearance of 
connective tissue ; h, cartilage-cells around its circumference, in different stages 
of development : in one the nucleus is seen undergoing division. 
