Degeneracy: Its Causes, Signs and Results

by Eugene S. Talbot

To appreciate the degeneracy observed in the ear its embryology requires study. Before the end of the first month[212] there appears around the external opening of the first gill-cleft (Fig. [35]) a series of six tubercles, two in front on the hind edge of the first visceral arch, one above the cleft and three behind it (Fig. [36]). A little later a vertical furrow appears down the middle of the hyoid arch, in such a way as to mark off a little ridge which joins on to tubercle 3 and descends behind tubercles 4 and 5. The second stage is reached by the growth of all the parts; the fusion of tubercles 2 and 3 and the growth of the ridge down behind tubercle 5 to become continuous with 6. After these changes it is not difficult to identify the parts.

FIG. 35.

FIG. 36.

Tubercle No. 1 is the tragus; 2 and 3, together with the arching ridge, represent the helix; 4, the anti-helix; 5, the anti-tragus; and 6, the lobule; the pit between the tubercles the fossa angularis. During the latter part of the second month the ear changes in its proportion somewhat in the irregular development. The third stage begins at the third month. The upper and posterior part of the concha arises from the surface of the head, and gradually but rapidly bends forward so as to completely cover the anti-helix and the upper portion of the fossa angularis. During this stage in mammals the assumption of the pointed form of the ear commences. The fourth stage begins at the fourth month, when the tubercles, which are now joined together by cartilages, commence to unfold and are completed by the fifth month. Finally, the sixth tubercle develops to form the lobule. This unfolding or development of the tubercles to produce the different portions of the ear and make it complete is not unlike the development of a flower from the bud. By this process may be understood how if, by malnutrition in one tubercle or bud, or should there be a larger supply of nutriment in one than another, malformation of the ear would result. If arrest of development of all the tubercles should take place at any period, from the first to the fifth month of fœtal life, the ear would resemble a semi-developed flower! (Fig. [37]).