TACTILE HAIR OF THE WHITE RAT 9 



ested is the terminal one. It passes through the infra-orbital 

 foramen and lying close against the maxilla and pre-maxilla 

 bones runs forward until just beneath the tactile hairs. 



In sections stained with osmic acid the nerve is seen to be a 

 typical cutaneous nerve consisting of many bundles of very dif- 

 ferent sizes and containing large and small fibers, fibers which 

 stain very black, paler fibers and fibers which are not stained 

 at all. It is a very large nerve in comparison with the branch of 

 the seventh which lies superficial to it. Even as it emerges from 

 the ganglion it has the form of a large flattened band and its 

 branches look like mere threads beside the main trunk. It is 

 estimated that this trunk as it leaves the infra-orbital foramen 

 contains from 15,000 to 20,000 fibers. Its great size corresponds 

 to the functional relations of the mouth parts which it serves. 

 In animals like the elephant, tapir, etc., or animals with strong 

 tactile hairs, its proportions are increased enormously. 



It often divides into ten or a dozen branches before passing 

 through the foramen, which is very much enlarged in many 

 rodents. The inferior portion of the anterior maxillary part of 

 the zygomatic arch is thin and flattened in rats, forming a verti- 

 cal fissure through which the infra-orbital nerve passes. The 

 superior part of the fissure is a rounded depression which at first 

 glance might be taken for the orbit itself. 



Soon after leaving the foramen this nerve anastomoses with 

 the infra-orbital branch of the facial, which is the motor nerve to 

 the same region. The fibers of the trunk thus formed are dis- 

 tributed to the skin of the upper lip and nose, but about half of 

 them go to the tactile hairs of which there are between forty and 

 fifty on each upper lip (fig. 3). These are arranged in six rows 

 with from five to nine hairs in each row, and there are often 150 

 or more large medullated fibers in the nerve bundle entering a 

 single foUicle of a large tactile hair. Usually, just before reaching 

 the root of the hair the nerve divides in two parts which pene- 

 trate the dermal sheath at the level of the lower third. One part 

 turns back, both divide many times and together they encircle 

 the follicle in a sort of palisade of longitudinally running fibers. 

 The further course of these fibers will be followed later (fig. 9). 



