GG 



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, 1 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 o 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 



HI 



PT V VI, 



FIG. 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 ; ar., articular end of ink., Meckel's cartilage ; I, maudibular 

 arch ; II, hyoid arch ; III, IV, 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.] 



