Organization and development of the preterminal nerve pattern in the palmar digital tissues of man

Palmar digital skin differs from the general integument of the human body in structure, nervous pattern and function. It is devoid of hairs, sebaceous glands and arrectores pilorum muscles, instead, it possesses a system of papillary ridges which contain several kinds of nervous receptors integrated in their structure (Cauna, '54). In the animal kingdom, pads of quadrupeds can be compared with the human palmar skin, but in most animals specialization of limbs for locomotion has reduced their finer sensory function and therefore, neurohistological observations based on the pads of animals cannot be directly applied to the digital skin of man. Nerve endings in human fingers have been intensely investigated during the last hundred years, but the preterminal organization and the development of the digital nerves have received surprisingly little attention. Vesalius (1565) in his neurological tables showed the digital nerves as plain branches of the two major nerves of the forearm. Vater (1741) was able to show microscopical ramification of these nerves, many of which ended in minute vesicles (Vater-Pacini corpuscles), but he did not detect plexus formation. Kolliker ( 1863) found that nerves entering the superficial fascia divided into branches and fascicles and formed extensive networks. These became most complex just deep to the papillary layer of the corium where true axonal divisions took place; in the papillary layer, nerve fibers ended in continuous loops (like capillaries). Meissner (1853) and Langerhans (1868) confirmed the existence of axonal divisions in the preterminal plexus, and the latter suggested that union of nerve fibers also occurred. Anatomists of later periods were generally in agreement with these observations. Stohr ('28) considered that the main significance of the plexus formation was to protect nerve fibers against mechanical damage during movements of the tissue layers, but in certain situations, e.g., in the pia mater or in the walls of blood vessels, the plexus constituted a mode of nerve termination wherein fibers lost their sheaths and ramified repeatedly producing an extensive neural surface which covered a large supply area. Stohr ('35) also considered that skm contained a system of neurosyncytium of non-medullated fibers in addition to the somatic plexus. This view was shared by a number of neurohistologists (Boeke, '40; Jabonero, '53; Seto, Fujii and Ikui, '54; Droz, '55; Hermann, '56; Sarter, ' 5 7 ) , but a number of workers have disputed the existence of such a syncytium (Pkrez, '31; Kirsche, '55; Abrahhm, '56). In recent times, investigations on the preterminal plexus were carried out by Pkrez ('31), Kuntz and Hamilton ('38) Weddell and his coworkers (Weddell, '41; Weddell, Pallie and Palmer, '54; Weddell, Taylor and Williams, '55) and Cauna ('56b). The development of the preterminal plexus has received little specialized attention. Pkrez and PCrez ('32) found that the subepithelial plexus in the human skin developed during the 6th fetal month before the dermal papillae were formed. Levi ('33) and Szymonowicz ('33) found that in fetal fingers, some nerve fibers reached the epidermis during the third month but