Michio Okamoto

Studies on the Human Triangular Tract of Helweg

Publisher Summary This chapter discusses the studies on the human triangular tract of Helweg. The triangular tract was first described as a fiber bundle that specifically existed in the spinal cord of psychopathic patients having emotional defects. In accord with the shape of its cross section at the level of the upper cervical spinal cord, the tract has been called “the Dreikantige Bahn of Helweg.” Detailed description of the tract is begins at the level of the CI segment, where the tract usually has its typical triangular shape in cross-section, thereby deserving the name of the triangular tract. It is located at the transition region between the anterior funicle and the lateral funicle at the superficial part of the spinal cord, thus making a flat triangle with its centrally directed tip and the base along the anterolateral surface of the spinal cord. The fiber tracts that are located in the area medial to the transition region between the anterior and posterior spinocerebellar tracts are customarily believed to include the spinothalamic tract, the spinotectal tract, and the spinoreticular tract, and they are a little later in myelination. It is found that the diffuse formation was very conspicuous in the area comprising the anterolateral aspect of the anterior horn, containing a great amount of unstained and stained extremely fine fibers.

On the commissura ganseri of the monkey

The study of a large number of dentitions representing all recent hylobatid species and the comparison of these data with fossil hylobatids as well as with fossil and recent members of the pongid and hominid families show that a progressive reduction of the third molars characterizes the phylogeny of all three hominoid families. Moreover, the details of the process of reduction appear to be similar in all three lines of evolution. 1. Reduction starts earlier in the upper than in the lower jaw. 2. In the upper M3, morphological reduction occurs prior to reduction in size. The latter becomes common only in living apes and in Middle Pleistocene hominids. 3. In the lower M3, reduction in size started apparently somewhat earlier than morphological reduction. However, the trend towards shortening of the third lower molar manifests itself extensively only during the last phase of hominoid evolution. Marked morphological reduction is even more recent. 4. There appears to be a broad correlation at any given time level between absolute size of the dentition and degree of reduction of the third molars, which may be but one aspect of the decrease in size of the entire dentition. The detailed similarity of the trends towards reduction of the third molars in the three hominoid families constitutes one of the best documented cases of parallel evolution in primate phylogeny.