172 
PROFESSOR A. M. PATERSON ON THE DEVELOPMENT OF 
the first splanchnic branch which joins the sympathetic does so just behind the origin 
of the vertebral artery. 
In older embryos the sympathetic cord presents similar characters, oiodified by the 
collateral development of other parts and organs. In Mouse embryos of 17-18 days 
(Plate 25, fig. 10, and Plate 28, fig. 15), in which the cartilages have become evident, 
the cord is still uniform and unsegmentcd. The cervical part of the cord is unal¬ 
tered ; the “ superior ” cervical ganglion terminates as before, in front and behind. 
Tlje cervical splanchnic branches do not join the sympathetic cord. The “inferior” 
cervical ganglion is joined by fine splanchnic brandies, along which cellular branches 
can be seen coursing towards the ventral roots of the spinal nerves. In the thorax 
the cord is uniform, diminishing in diameter from before backwards. In transverse 
sections it is oval at the points where the intercostal arteries cross it, round in inter¬ 
mediate situations. The splanchnic branches arise from both spinal nerve roots, and 
in the anterior part of the thorax appear to join the sympathetic cord entirely; in 
the hinder part of the thorax they partly join, and partly pass, the cord. Cellular 
branches can be traced ventrally from the sympathetic, as well as others, which 
pass centrally along the splanchnic branches of the nerves. The whole thoracic 
portion appears much more minute than in younger embryos, on account of the 
great strides which other parts have taken in development. It narrows gradually 
to the level of the diaphragm, and as it passes through this it becomes very minute. 
Behind the diaphragm, and in relation to the liver, the cord is greatly increased in 
thickness, and gives off considerable bundles, partly fibrous, partly cellular, to form 
the solar plexus, suprarenal body, &c., and derived partly from the splanchnic 
branches directly, partly from the sympathetic cord. Behind that the cord again 
gradually narrows; the splanchnic branches in some sections wholly (as in fig. 10) 
join the cord ; in others can be seen passing alongside it towards the mesentery, 
accompanied by a cellular stream from the sympathetic. In the lumbar region the 
sjilanchnic branches are very long and fine ; they arise from both spinal nerve roots. 
They cease behind at the bifurcation of the aorta. None were found behind that 
point. Behind that level the cord gradually tapered oft* and lay in relation to the 
middle sacral artery, behind which, near its termination, a junction seemed to be 
effected between the cords of tiie two sides. 
The connexions of the spinal nerves and sympathetic cord have now been suffi¬ 
ciently described. First a cellular, unsegmented column, the sympathetic, appears on 
each side of the middle line in the neighbourhood of the aorta. This is, secondarily, 
joined by branches from the inferior primary divisions of certain spinal nerves, which 
have a definite origin and distribution. They are fundamentally spinal nerves, 
deriving their origin from both dorsal and ventral roots, and being formed by the 
division of the inferior primary division into two branches, onesomatic, the other (the 
nerve in question) splanchnic. Coursing downwards and inwards, they reach the 
sympathetic cord. Here the splanchnic branch terminates in one of two ways. It 
