Herrick, Commissura Infima. 413 



In Catostomus the general plan is much as in Cyprinus, but 

 more compact. The visceral commissural nucleus (nucleus of 

 Cajal) is not well developed in the median line, but is a paired 

 structure lying close to the meson at the ends of the commissura 

 infima, whose visceral portion is short, thick and more compact 

 than in the carp. The descending sensory vagus root is very 

 large and most of its root fibers decussate in this commissure. 

 The visceral commissural nucleus is not as clearly separate from 

 the surrounding structures as in the carp. This nucleus and the 

 adjacent parts, both forward and backward, are connected by a 

 strong tract of unmeduUated external arcuate fibers with the funic- 

 ulus ventralis. The nature of this connection I have not been 

 able to make out. It is present in Ameiurus and contains some 

 feebly medullated fibers. The dorsal end of this tract is shown in 

 fig. 6 of my recent paper on Ameiurus (1906), passing down later- 

 ally of the tracts marked /. /. It seems to be the path from the 

 visceral sensory centers to the motor nuclei of the muscles of the 

 fins and trunk. 



The great enlargement of the visceral centers of cyprinoids is 

 confined to the medulla oblongata. The spinal visceral centers 

 are even smaller than in some other fishes, and the commissural 

 nucleus cannot be traced far back into a clearly defined area of 

 the spinal cord. 



The somatic portion of the commissura infima is less compact, 

 being but little different from the dorsal commissure of the spinal 

 cord, which is well developed. The somatic commissural nucleus 

 is developed about as in Ameiurus. One of its neurones is shov\'n 

 in fig. 5. 



The gold fish, Carassius auratus, resembles the carp very closely, 

 the chief difference being the greater distinctness of the visceral and 

 somatic components of the commissura infima. The tractus 

 funiculo-ambiguus is well developed and there is a conspicuous 

 well medullated descending tract from the visceral commissural 

 nucleus, which passes directly back from the commissure close to 

 the median line. It passes caudad among the transverse fibers 

 of the somatic commissura infima and disappears before reaching 

 the caudal end of the funicular nuclei. This tract is present in 

 all of the fishes which I have examined, but is exceedingly variable 

 in size. Usually it is unmeduUated. Golgi sections of various 

 species have shown sparse fibers passing from it ventro-late rally 



