ANATOMY OF A 17.8 MM. HUMAN EMBRYO 65 



tends ventrally between the vagus and the internal jugular vein. 

 At the ventral border of the vagus, a little caudad of the termina- 

 tion of the linguo-facial vein, it gives off its ramus descendens. 

 The latter extends caudad, ventral to the internal jugular vein, 

 and joins the descendens cervicalis to form the ansa hypoglossi. 

 The hypoglossus then continues cephalad, medial to the linguo- 

 facial vein and the developing submaxillary gland (plate 5) to 

 divide in the tongue into muscular branches. 



Spinal nerves 



Plates 1 and 4 show portions of the ventral divisions (rami 

 anteriores) of the spinal nerves (Nn.sp.); plate 3 gives a lateral 

 view of the right cervical nerves and plexus; and plates 2 and 5, 

 a lateral view of the left cervical nerves and plexus. In plates 5 

 and 6 the relations of these nerves to the jugular lymph sac 

 (S.jug.) are shown. It will be noted that some of the ganglia 

 are still connected by ganglionic bridges. 



Nn. cervicales. The cervical nerves divide just beyond the 

 union of the dorsal and ventral roots into dorsal and ventral pri- 

 mary divisions (rami posteriores and anteriores). The dorsal 

 rami are shown as stumps with the exception of the great occipital 

 branch {N.occ.m.) from the second, a piece of which has been 

 added. The ventral primary divisions run ventrally and cau- 

 dally, and the second to the sixth inclusive, extend laterally to 

 the vertebral artery {A. vert.). 



The first cervical nerve has a long slender ganglion (plates 2 

 and 3), the dorsal part of which overlies the accessory nerve 

 (N.acc). The ganglion shows signs of atrophy in its middle 

 part, especially on the left side. As the nerve extends ventrally 

 it is in close contact for a distance with the vertebral artery (plates 

 1 and 2), which it crosses on the medial side. A short communi- 

 cating branch connects the first cervical nerve with the ventral 

 primary division of the second cervical. The fibers of this 

 communicating branch join the hypoglossal nerve, and are thought 

 to assist in forming the ramus descendens hypoglossi. 



The ventral primary division of the second cervical nerve be- 

 haves differently on the two sides. On the left (plate 2) beyond 



THE AMERICAN JOUENAL OF ANATOMY, VOL. 17, NO. 1 



