The mandibular or Meckel's cartilage is continued up into the tympanum where it joins the proximal end of the cartilage of the second or hyoid arch, and it is from this junction (hyomandibular plate) that, according to H. Gadow, Anat. Anzeiger, Bd. 19, p.
396, the malleus and incus bones of the middle ear are developed (see EAR). Between the slender process of the malleus and the region of the inferior dental fora men, the cartilage later on disap pears and its fibrous sheath forms the long internal lateral or sphe nomandibular ligament (see fig. 33, L.I.L.).
Each half of the lower jaw was long considered to be composed of several distinct skeletal ele ments, homologous with the ele ments found in the jaws of lower vertebrates, but it seems evident that in man the process of ossi fication is slurred over although some of the original elements of the lower vertebrates are repeated as temporary cartilaginous masses, e.g., coronary, condylar and angular. (See A. Lowe, "Development of Lower Jaw in Man," Proc. Anat. Soc. of the University of Aberdeen, 1905, p. 59.) At birth the two halves of the mandible are separate as they are throughout life in many mammals (e.g., rodents), but in man they join together about the end of the first year.
It has been stated that within the tympanum the dorsal or proximal ends of the first and second visceral arches unite to form the hyomandibular plate from which the malleus and incus are derived. The stapes is also probably formed from the proximal end of the second or hyoid arch (see fig. 33, St.), and just ventral to this the cartilage of the arch fuses with that of the periotic capsule, where it is later on ossified as the tympanohyal element of the temporal bone (fig. 33, T.H.). From this point the cartilage
becomes free from the skull and runs round the pharynx until it meets its fellow of the opposite side in the mid-ventral line. That part of the cartilage which is nearest the skull remains as the stylohyal element (fig. 33, S.H.) and this later on ossifies to form the styloid process which fuses with the tympanohyal between twenty and twenty-five. For some distance beyond the stylohyal element the cartilage degenerates into fibrous tissue forming the stylohyoid ligament; this represents the epihyal element, and oc casionally instead of degenerating it ossifies to form an abnormal bone (fig. 33, E.H.). Near the middle line the cartilage persists as the ceratohyal element or lesser cornu of the hyoid bone (fig. 33, C.H.), while the most ventral part, where it fuses with its fellow of the opposite side as well as with the ventral part of the third arch, is the basihyal or body of the hyoid bone (fig. 33, B.H.).
The dorsal part of the cartilage of the third arch is wanting, but the lateral part forms the thyrohyal or great cornu of the hyoid bone (fig. 33, Th.H.), while its ventral part fuses with its fellow of the opposite side as well as with the ventral part of the second arch to form the body of the hyoid bone. The fourth and fifth arches only develop cartilage in their ventro-lateral parts and fuse to form the thyroid cartilage of the larynx (fig. 33, Th.C.) (see