298 Geora-e L. Streeter 



b"- 



At the same time that the dorsal and ventral roots imite to form the 

 main trunk they give off lateral fibres to form the dorsal branch, the 

 so-called posterior primary division, which turns back dorsalward and 

 covers in the distal part of the ganglion, and breaks up among the cells 

 which are to form the long muscles of the back. 



The remainder of the nerve trunk is continued forward as the ventral 

 branch or anterior primary division. From its median side there is 

 given off the ramus communicans, which extends medianward to the 

 region of the aorta and ends in the sympathetic ganglion cord. The 

 rami communicantes and the sympathetic cord are not shown in Plate 

 I. The main trunk terminates in two branches, the anterior and lateral 

 terminal branches, which correspond to the anterior and lateral cutaneous 

 branches of the adult, and which in the thoracic and abdominal regions 

 end among the cells giving rise to the musculature of the front and 

 lateral body wall. 



Throughout the spinal region there is a tendency for the nerve trunks 

 to unite at the level of the lateral terminal branches and form inter- 

 segmental loops. This loop — or plexus — formation may involve either 

 the lateral or the anterior terminal branches, or both. "We thus have 

 produced the cervical, brachial and lumbosacral plexuses. 



In the cervical region the anterior and lateral terminal branches form 

 two separate plexuses; the former produces the deep cervical plexus 

 and the latter the superficial cervical plexus. The superficial cervical 

 plexus consists of the union of the lateral terminal branches into loops 

 from which are given off the cutaneous branches to the auricular, cervi- 

 cal and occipital regions. The deep plexus results in the formation of 

 the ansa hypoglossi and the phrenic nerve. The former is produced by the 

 fusion of the second and third cervical nerves into the descendens cervicis, 

 which unites in a loop with the hypoglossal, together with which the 

 first cervical has been incorporated above. From this loop are given 

 off the short branches which end among the cells that are to form the 

 hyoid musculature. The main trunk of the hypoglossal bends sharply 

 medianward to end in the tongue anlage. The deep cervical plexus 

 was studied in an older embryo (14 mm. — Mall embryo No. 144) and 

 described in a former paper (Streeter, '04). The ansa hypoglossi is 

 shown in Fig. 11 and is essentially the same as in the present case, 

 differing only in that the communication between the first cervical and 

 the hypoglossal could not be traced. In Plate II of that paper the r. 

 descendens is labelled wrong; the leader should extend to a point above 



