404 



ELEMENTAR Y ANA TOMY. 



[LESS. 



tially consists of all those branches of the cerebro-spinal 

 system which originally pass down to the viscera in the 

 inner plate of each bifurcating visceral lamina of the embryo, 

 and which are represented in the adult by branches similarly 

 extending into the mesentery, 1 and by their serial homologues. 



FIG. 352. DIAGRAM OF THE DE- 

 VELOPMENT OF THE TRUNK AND 

 ITS SKELETON, as shown in a section 

 made at right angles to the trunk's 

 long axis. 



iic, neural canal ; ex, epaxial cartilages 

 ascending to surround it ; px, par- 

 axial cartilages descending in the 

 plate, or layer {Ive), external to pp, 

 the pleuro-peritoneal cavity ; lvi t in- 

 ternal plate of the split ventral 

 lamina. 



FIG. 353. DIAGRAM OF THE FURTHER 

 DEVELOPMENT OF THE TRUNK, as 

 shown in a section similar to the last. 



a, alimentary canal supported by a me- 

 sentery formed of the dorsal portion 

 of the inner parts of the split wall of 

 the embryonic ventral laminae ; e, 

 epaxial arch ; k, hypaxial arch de- 

 scending in the median line in the 

 root of the inner part of the split wall 

 of the ventral laminae ; /, rib, bi- 

 furcating proximally and abutting 

 ventrally against the sternum, which 

 thus completes the paraxial arch ; in, 

 peritoneum, bounding on all sides //, 

 the pleuro-peritcn ;al space. 



Moreover, these branches are connected together on each side 

 (near their origins) by commissural nerve-fibres, and it is 

 these two linear series of commissures (enlarged into ganglia 

 at their successive points of junction) which constitute the 

 two longitudinal gangliated cords before spoken of. 



It is the presence of these commissural cords which makes 

 so marked a contrast between the sympathetic system and the 

 other branches of the spinal nerves, which in man exhibit no 

 signs of such continuous longitudinal union. But an extended 

 survey shows us that these latter (the undoubted spinal nerves) 

 may also be connected together by similar axially extended 

 commissural cords, for such do exist in the class of Pushes. 



The sympathetic nerves of the viscera present three great 

 complications, termed (i) the cardiac plexus, (2) the solar 

 plexus, and (3) the hypogastric plexus : these are placed respec- 

 tively above the heart, behind the stomach, and in the pelvis. 



1 See Lesson XI., p. 458. 





