420 Journal of Comparative Neurology and Psychology. 



spinal cord, which probably sends some fibers into the dorsal com- 

 missure of the spinal cord. If the nucleus dorsalis (Clarke's 

 column) of the spinal cord represents the visceral sensory center 

 of the spinal cord, as supposed by some recent anatomists, then the 

 spinal root fibers which cross in the dorsal commissure to termi- 

 nate in the heterolateral nucleus dorsalis are comparable with the 

 sensory root fibers of the vagus which cross in the commissura 

 infima. But our knowledge of the visceral sensory system in the 

 spinal cord is still too imperfect to permit of final decision of any of 

 these questions. The somatic component of the commissura 

 infima is no doubt comparable with the somatic sensory fibers 

 which make up the greater part of the dorsal commissure through- 

 out the spinal cord. 



It is probable that in fishes (and in higher vertebrates) visceral 

 centers which primitively were arranged, like the somatic centers, 

 in a metameric way throughout the spinal cord have become con- 

 centrated in the medulla oblongata, the intestinal branch of the 

 vagus and various sympathetic connections of the cranial nerves 

 having to a large extent supplanted the original segmental visceral 

 nerves of the spinal cord. If, as appears probable, the primitive 

 vertebrate ancestor had gills extending down the greater part of 

 the length of the trunk region, as in Amphioxus, the explanation 

 for this concentration of the visceral nerves is clearly apparent in 

 the progressive loss of the more caudally placed gills as we ascend 

 the vertebrate series. 



In the extensive region of the enlarged visceral sensory area of 

 the medulla oblongata the roof plate is membranous and widely 

 extended laterally. This feature prevents the visceral sensory 

 commissural fibers of the medulla oblongata from crossing at their 

 own level and necessitates their passage caudad to the region 

 behind the calamus scriptorius, where they are concentrated as the 

 visceral commissura infima. Both root fibers of the vagus and 

 secondary visceral tracts from the vagal lobes are involved in this 

 movement, which has carried with it a certain number of cells per- 

 taining to these commissural fibers. These cells make up the 

 visceral commissural nucleus and are probably mainly terminal 

 nucleus cells for the vagus root fibers of the commissure. The 

 secondary connections of this nucleus are substantially the same 

 as those of the visceral sensory nucleus of the vagus in the medulla 

 oblongata. 



