The orbit begins in the large-cell part of the nucleus ruber and crosses directly over the mean line. It reaches in humans, where it is almost non-existent with unclear function, only down into the cervical spinal cord.
Compared with lower mammals, which have a well-developed tractus rubro-spinalis extending throughout the spinal cord and with extensive contacts in the internal and external motor neuron group, it appears that the tractus cortico-spinalis in humans has achieved its size and importance at the expense of the tractus rubro-spinalis.