The body in turn becomes further attenuated to form the tail which courses along the roof of the temporal horn of the ventricle and terminates at the amygdaloid nucleus. The PU and CD nuclei are incompletely separated by the internal capsule. This is a phylogenetically new component of the brain. It appears in the reptilian brain and forms the largest part of the corpus striatum in mammals. Histologically, the structure of the CD and PU are similar but not homogenous.
Neurons are basically of two basic types, projection neurons and interneuons.
Other neurons have dendrites that lack spines and serve an associative or interneuronal function. The GP is situated medial to the putamen and is seperated from it by a thin lamina of myelinated fibers called the external medullary lamina. The GP is traversed by numerous myelinated fibers which give it its characteristic pale appearance in stained sections and from which its name derives.