Johnston, Nerves of Petromyzonts. 591 



internal and external gill openings as above described. Other 

 fibers innervate the muscle of the anterior wall of the gillsac. The 

 nerve then gives off the branch to the m. hyo-hyoide us anterior, 

 mentioned above and moves onto the postero-internal surface of 

 the gill muscle farther from the middle plane than before (fig. 9). 

 Here it continues to give off branches to the lining of the gill sac 

 and to the muscle until it reaches the lower border of the gill sac. 

 Here it turns ventro-mesad and runs caudad on the surface of 

 the inferior jugularvein (figs. loandii). The course of this termi- 

 nal portion of the VH nerve is beautifully impregnated in one 

 transverse and one horizontal series of sections. The transverse 

 sections include only the first two gill sacs and the horizontal sec- 

 tions the first three sacs, and the nerve continues to the end of 

 the series in each case. As it goes backward it gives oflF branches 

 laterad to ramify on the ventral wall of each gill sac and branches 

 mesad to the walls of the blood vessels and to the thyroid gland. 



From this account it is seen that the visceral sensory portion of 

 the VH nerve supplies the lining of the anterior half of the first 

 gill sac. Since the hyomandibular sac, which lies in the embryo 

 in front of the arch in which the VH nerve runs, is aborted, the 

 branchial portion of the nerve corresponds to the posterior 

 branchial branches of the IX nerve. The continuation of the 

 nerve caudad beneath the gill sacs will be considered farther under 

 the head of the sympathetic system. 



The supply of fibers from all the branchial nerves to the lining 

 of the gill sacs, lamellae and filaments is very rich indeed. Great 

 numbers of medium and fine fibers interlace beneath the gill 

 larnellae and send up fine fibers along the filaments to end between 

 the epithelial cells. It is very difficult in a drawing on one plane 

 to give an adequate idea of the richness of the nerve supply to 

 the gills. The lining of the sac also is everywhere very richly 

 supplied with fibers. 



THE VISCERAL MOTOR COMPONENTS. 



The motor fibers in the X, IX and VII nerves supplying the gill 

 sacs have been described in a general way in the above para- 

 graphs. As the nerve descends in its branchial arch it gives motor 

 fibers to the half of each adjacent gill sac. The muscle in the 

 anterior wall of the gill sac is much thicker than that in the pos- 



