386 THE RABBIT. 



early established, and are all present by the eleventh day (Fig. 

 165) . The nerve trunk, beyond the ganglion, divides at once into 

 a smaller dorsal branch, NN' ; and a larger ventral branch, NN ; 

 each branch containing nerve fibres from both the dorsal and 

 ventral roots of the nerve. The dorsal branch, NX', runs out- 

 wards and upwards, to the muscles and skin of the back ; while 

 the ventral or larger branch, NN, runs downwards in the body 

 wall, and in the somites opposite the limbs sends branches into 

 these latter. 



6. The Sympathetic Nervous System. 



This has received much attention, but several points con- 

 cerning its earl}' origin still remain in doubt. Before the end 

 of the eleventh day it is already well established (Fig. 165, NY), 

 consisting of a main ganglionated cord running along each side 

 of the body, close to the dorsal surface of the aorta, and receiving 

 branches from the ventral branches of the several spinal nerves 

 as it passes these. Dr. Paterson, from observations chiefly on 

 rat embryos, but partly on rabbits, concludes that the longi- 

 tudinal cord is the first part of the sympathetic system to be 

 developed, that it arises in the mesoblast entirely indepen- 

 dently of the spinal nerves, and is at first devoid of ganglia ; he 

 believes that the connection of the longitudinal cord with the 

 spinal nerves is a secondary one, and is effected by outgrowths 

 from the ventral branches of the spinal nerves after the longi- 

 tudinal cords are established. These observations are, however, 

 so entirely at variance with what is known as to the develop- 

 ment of the sympathetic nervous system in other Vertebrates, in 

 which the sympathetic develops merely as a specialised portion 

 of the spinal nervous system, that it seems preferable to suspend 

 judgment on the matter, pending renewed investigations. 



The connections of the spinal nerves with the longitudinal 

 sympathetic cords are limited to the thoracic and lumbar 

 regions, but the cords themselves extend forwards along the 

 neck to the head, where they acquire connections with the 

 hinder cranial nerves. 



7. The Supra-renal Bodies. 



The supra-renal or adrenal bodies, which in the adult rabbit 

 form a pair of small, round, yellow bodies, a little way in front 

 of the kidneys, are developed from two distinct sources. 



