248 COMPARATIVE ANATOMY 



The sympathetic, accompanying the arterial trunks, extends 

 along the vertebral column and passes anteriorly into the 

 skull, where it comes into relation with a series of the cerebral 

 nerves (cf. pp. 237, 241 and Fig. 180) similar to those which it 

 forms further back with the spinal nerves. 



The original segmental character frequently disappears later on, 

 and this is especially the case in those regions where marked 

 modifications of the earlier metameric arrangement of the body 

 have taken place viz., in the neck and certain regions of the trunk, 

 especially towards the tail : thus in Mammals there are never more 

 than three cervical ganglia. 



Nothing is known of a sympathetic in Amphioxus. In 

 Petromyzon typical ganglion-cells occur more or less sparsely along 

 the dorsal and ventral spinal nerves in the lateral walls of the body. 

 Nests of cells are present more frequently in the region where the 

 parietal veins open into the cardinal veins than alongside the aorta, 

 and they also occur along the caudal vein and its branches : these 

 ganglia are connected with the suprarenal organ (q.v.}. The 

 sympathetic extends into the head. 



In Elasmobmnchs the sympathetic reaches a higher stage of 

 development, and it has been shown that the ganglia first appear 

 after the dorsal and ventral roots have united to form the spinal 

 nerve-trunks, just at their point of union, each ganglion containing 

 from the first both afferent and efferent elements. Except in 

 its most anterior embryonic segment, in which the ciliary ganglion 

 represents a part of this system, the head is without sympathetic 

 ganglia. A sympathetic cord, connecting the ganglia, is not de- 

 veloped in Elasmobranchs, although some of the individual ganglia 

 may become united together, while others disappear at an early 

 stage. 



A cranial portion of the sympathetic exists in Tcleosts, arising 

 from the trigemino-facial system of nerves and possessing three 

 ganglia : in the trunk, too, there is a well-developed cord of ganglia, 

 frequently connected with its fellow by transverse commissures, the 

 two cords gradually converging antero-posteriorly. A similar con- 

 dition has been found to occur in the Dipnoi (Protopterus), in which 

 the delicate longitudinal sympathetic cords, with occasional ganglia, 

 extend along the aorta and notochord : nothing is known of 

 their connection with the cerebral nerves. 



In Amphibians (Fig. 145), the sympathetic reaches a high stage 

 of development. It ends anteriorly in the ciliary ganglion, extends 

 along the aorta through the trunk and caudal regions as a 

 ganglionated cord, and has numerous anastomoses with the spinal 

 and cerebral nerves ; it is intimately related with the suprarenal 

 and abdominal veins (postcaval and revehent renal veins). 



In the Sauropsida the cervical portion of the sympathetic is 

 usually double, one part running within the vertebrarterial canal 

 alongside the vertebral artery, In all other Vertebrates the whole 



