66— THE STRUCTURE OF MAN 
cornu (III), which is paired and projects therefrom backwards. 
The body may be regarded as the basal element of the second and 
third embryonic skeletal arches,’ while the posterior cornua repre- 
sent the lateral elements of the third (or first branchial) arch 
alone (cf. Figs. 45, 46, and 107). 
In the earliest stages of the embryo, the ridge which will 
afterwards develop into the second or hyoid visceral arch, sends 
a process backwards, which covers a deep groove (the cervical 
groove) on the postero-lateral edge of the cephalic region. The 
third and fourth branchial arches lie in the hollow thus - 
formed, and they gradually cease to be externally evident. 
The entrance to this cervical groove is bounded by the hyoid 
uf It It TV. Vo aa 
Fic. 46.—SKULL OF A TAILED AMPHIBIAN (Menopoma). The skeletal arches are 
lettered serially with those of Man, in Figs. 45 and 105. 
qu., quadrate cartilage ; a7., articular end of imk., Meckel’s cartilage ; I, mandibular 
arch ; II, hyoid arch; JII, 1V, V, VI, branchial skeletal arches. 
arch ; and there can be little doubt that we have in the above- 
mentioned ridge a feeble homologue of the gill-cover of fishes 
and metamorphosing Amphibia. It at a later stage fuses with 
the adjacent body wall, the cervical groove (branchial chamber 
of the Anamnia) becoming thus closed. 
The hyoid apparatus, which is intimately connected with the 
cervical, lingual, and mandibular musculature, is in fibrous con- 
nection (thyro-hyoid ligament) with the upper edge of the 
laryngeal skeleton’; and of this skeleton the thyroid cartilage at. 
least (IV, V, Fig. 45) arises from the fourth and fifth bran- 
chial arches (cf. Fig. 107 and p. 151). 
1 [It is usually stated to be ossified from a single centre in Mammals, but the 
fact, to which my friend Mr. M. F. Woodward has drawn my attention, that it may 
be occasionally subdivided by a transverse suture into two portions (ex. Lepus) 
indicative of its ossification from two recurrent centres, is of much interest in this 
connection.—G. B. H.] 
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