THE SYMPATHETIC NERVOUS SYSTEM IN MAMMALS. 
173 
may end entirely in the sympathetic cord, as seems to occur in tlie anterior part of 
the thoracic region. [This mode of ending is not absolutely certain. It may be only 
apparent from the manner of section of the nerve trunks.] On the other hand, in the 
hinder dorsal and lumbar regions, the splanchnic branch on reaching the cord divides 
into two parts, one of which joins, the other passes beyond the cord. In either case the 
splanchnic fibres which join the cord become directly connected with the component 
cells. The connexion between the spinal nerves and sytnpathetic cord is, therefore, to 
be regarded as quite a secondary connexion. 
In certain regions no such connexions can be made out. Behind the kidney and 
bifurcation of the aorta (fie., behind the lumbar region) the splanchnic branches 
cease. In front of the fore limbs and the origin of the vertebral arteries (fie., in the 
cervical region) the splanchnic branches do not join the sympathetic. In comparatively 
advanced embryos distinct nerves, analogous to the splanchnics, course inwards round 
the vertebral artery to the tissues lying round the growing vertebrae and beneath the 
spinal cord, but occupying a position altogether dorsal to the sympathetic cord and 
unconnected with it. 
The regions, in short, where the sympathetic cord is joined by the splanchnic 
branches are those in which it lies in relation to the main vascular trunks—in the 
line of the embryonic aorta. 
3. Formatioyi of the Sympathetic Ganglia. 
Up to the time of the formation of the cartilaginous vertebral centra there is no 
gangliation or constriction of the main sympathetic cord. Sagittal sections of embryos 
prior to this date have already been referred to and their characters noted. These 
are exemplified in Rat embryos of 8 days (Plate 26, fig. 11), of 10 days (fig. 12), 
of 15 days (Plate 27, figs. 14 and 14a), &c. 
In sagittal sections of Mouse embryos of 17-18 days (Plate 28, fig. 15) the 
vertebral centra (F./S.) are distinctly cartilaginous, connected together by dense 
masses of cells, and plainly demarcated from the surrounding tissues. The segmen¬ 
tation of the body is also indicated by the intercostal vessels [Int.) lying opposite to 
the vertebral centra. Along the sympathetic cord there are seen at places, in 
successive series of sections and in relation to these segments, occasional bulgings. 
These are rare, however; sections through a considerable length of the cord at its 
thickest part show it as an unconstricted column {Sy., fig. 15), composed of large 
fusiform cells with terminal processes, and with large, deeply-stained nuclei. 
Sagittal sections of a Human embryo, about fin. in length, hardened in spirit, and 
stained with aniline blue-black, show a slightly later condition (Plate 30, fig. 19). 
They are very instructive, as indicating the manner in which the gangliation of the 
sympathetic cord occurs. The several jmrts of the vertebrae are already moulded in 
cartilage, the intervals between the centra being filled up as before with embryonic 
cellular tissue. When serial sections of this embryo are examined, the first indication 
