22 The Sympathetic Nervous System in Mammals. [May l r 



verse cellular communications take place on the dorsal aspect of the- 

 middle sacral artery between the cords of opposite sides to produce 

 the ganglion impar and the loop of connexion between the caudal 

 ganglia. 



The peripheral branches from, the sympathetic cord arise as cellular- 

 buds or outgrowths which are first seen about the time when the 

 splanchnic branches of the spinal nerves join the cord (mouse, rat, 

 11 12 days; human embryo 1st month). They accompany the 

 parts of the splanchnic branches which do not join the sympathetic 

 cord into the splanchnic area ; and, especially in the hinder thoracic- 

 region, form considerable masses traceable along the main vessels, 

 which in older embryos give rise to parts of the splanchnic nerves, 

 as well as the medullary portions of the suprarenal bodies, as previous 

 observers have described. 



The gray rami communicantes may (doubtfully) be said to belong- 

 to the category of peripheral branches from the sympathetic cord. 

 They appear to arise from the cord as cellular outgrowths which 

 pursue a centripetal course along the splanchnic branches of the- 

 spinal nerves towards their roots ; but in regions where these are 

 absent, or are unconnected with the sympathetic cord, I have not 

 been able to satisfy myself about their formation. 



The principal conclusions derived from these investigations are 

 that the sympathetic system in Mammals is mesoblastic, is formed in 

 situ out of the cellular tissue surrounding the embryonic aorta, and 

 is at first entirely independent of the cerebro-spinal nervous system : 

 it is primarily uniform and unsegmented, in this respect resembling 

 the organs in the splanchnic area the vascular and alimentary sys- 

 tems with which it is so closely related, functionally as well as 

 structurally. It becomes secondarily connected with certain spinal 

 nerves by the growth from the latter of the white rami communi- 

 cantes, and in consequence becomes gangliated in an irregular manner.. 

 From the main cord cellular outgrowths arise which form peripheral,, 

 non-medullated nerves, plexuses, and ganglia, as well as the medullary 

 portions of the suprarenal bodies. 



Morphologically, the Mammalian sympathetic cord resembles the- 

 structures with which it is in structural and functional relation, in 

 being mesoblastic, and in its development primarily unsegmented. It 

 is a rod, fibro-cellular in structure, out of which, on the one hand,, 

 are produced certain ganglia and nerves, and which, on the other 

 hand, becomes connected with certain cerebro-spinal fibres the- 

 splanchnic branches of the spinal nerves. The mechanism thus pro- 

 duced may be regarded as providing for the guidance of these fibres 

 (e.g., vasomotor nerves) to their destinations, and as regulating their 

 proper distribution to the vascular and alimentary systems. 



In conclusion, I wish to express my indebtedness to Professor- 



