NERVES OF THYROID AND PARATHYROID BODIES 95 



being especially true of the final end fibrillae. In fact these 

 nerves appear as if consisting of many irregular enlargments con- 

 nected by fine threads (fig. 4). The varicosities may be only 

 slightly larger than the fibers or they may be twice as large; the 

 larger ones are cylindrical, spherical, or irregularly triangular, the 

 majority, however, are either oval or spindle-shaped. 



The final endings of the nerves consist of fine end branches. 

 The smallest fibers usually divide into two, sometimes more, end 

 fibrillae. The course of these fibrillae is very irregular, they are 

 very varicosed and soon end. The tip consists of a spindle- 

 shaped enlargement or end-knob. This ending sometimes occurs 

 in the perivascular connective tissue, sometimes in the tunica 

 adventitia, but most often in the tunica media in relation to the 

 smooth muscle fibers of the vessel walls. Occasionally endings 

 from the perivascular plexuses can be seen penetrating the peri- 

 vascular connective tissue and terminating in relation with the 

 bases of the epithelial cells of the immediately adjacent follicles. 



The plexus surrounding the arteries is more dense and com- 

 plex than that surrounding the accompanying veins. The venous 

 plexus resembles the arterial in formation and architecture but 

 it is not nearly as elaborate. The peri-capillary plexus is usually 

 formed from the few end branches of a single fiber, running par- 

 allel to the course of the vessel. 



The most important nerves of the thyroid from a physiological 

 standpoint are those that have their endings in intimate relation 

 to the gland cells, for they are the true glandular or secretory 

 nerves. These nerves pass into the gland substance alongside 

 of, or in the walls of the arteries, together with those that form 

 the perivascular plexuses and furnish the vaso-motor supply. 

 They assist in the formation of the perivascular plexuses but there 

 is no way of distinguishing them from the vaso-motor nerves. 

 There may be, however, a physiological difference in the two 

 kinds of nerves, but if such exists, it cannot be determined histo- 

 logically. 



As has been said before, the nerves do not leave the large arte- 

 ries and pass directly into the gland substance, for this does not 

 occur until the vessels, by branching, have decreased very much in 



