The thyroid gland is one of those organs, commonly known by the name of ductless or blood-glands, about which our knowledge is still in a very unsatisfactory condition. The function of this body is quite unknown, and its minute structure is still involved in some obscurity. Results of previous Investigations.—The present state of our knowledge of the structure of this organ may be gathered from the writings of Henle (‘Handbuch der systematischen Anatomie des Menschen,’ 1866, vol. ii. pp. 535-541), Kölliker (‘Handbuch der Gewebelehre,’ 5th ed. 1867, pp. 480-482), Verson (Stricker’s ‘Human and Comparative Histology,’ Sydenham Society’s translation by H. Power, vol. i. pp. 370-372), and Frey (Das Mikroskop, 4th ed. 1871, pp. 232 & 283).