SOME OF THE SUB AXIAL AKCHES IN MAN. 
123 
be detached, and grows downwards and forwards to meet the corresponding structure 
from the opposite side in the middle line (Plate I. fig. 2, fig. 3, th. h). Although 
it is difficult to assign to it any particular function, it may be noticed that it is in rela- 
tion with the respiratory tract in its development, and is virtually a rudimentary bran- 
chial arch*; and further, that it forms a girdle beneath which the air-tube is continued, 
and perhaps guided, as the trachea into the thorax. 
The Trachea. 
Whilst the cartilages of the larynx are acquiring their permanent characters, the tra- 
chea is formed by a continuous growth from the membrane below the cricoid cartilage. 
I have not as yet definitely traced out the formation of the bronchi ; there is, I believe, 
no doubt but that they result from the division of the trachea. 
The Seventh or Exoccijpital Arch — The Shoulder-girdle. 
At first it occurred to me that this should be termed a subaxial arch, and it still 
seems desirable to number it as following the sixth or laryngeal outgrowth of membrane 
from the base of the skull. But exception might be taken to the term subaxial, as 
defined to refer to the position of an arch with reference to the notochord, when applied 
to this structure ; for the membrane from which the clavicle and scapula take origin 
grows out from the cartilage of the base of the skull in a line from the extremity of the 
membrane for the hyoid and laryngeal arches, below the lingual arch, and all along the 
side and hind plate in which the exoccipital and supraoccipital ossifications commence. 
From these the membrane of this arch is separated with some difficulty, leaving a rough 
edge. Thus whilst the ^membrane for the hyoid and laryngeal arches grows from the 
basioccipital region and is subaxial, that for the shoulder-girdle extends from the 
lateral and posterior portions of the occipital cartilage, having also for a short distance 
relations with the cartilage for the structures connected with the internal ear; it there- 
fore is best described as an exoccipital arch, growing from the cartilage of the base of 
the skull, but at the side of and behind the axis line represented by the notochord. 
The growth of this membrane is extremely simple. Dissecting at about the thirty- 
fifth day (foetus -45), it is seen as a fold of membrane extending from the root of the 
mandible on either side downwards, its connexion with the mandible being formed by 
* With reference to this point I would refer to the remarks which I made when examining the relations of 
this body. “ One cannot but be attracted by this connexion with the trachea, on which tube the thyroid (even 
if it he not developed from the membranous air-tube) buds and attains some little size, a formation reminding 
one of that of the lungs coming out from the front wall of the oesophagus, that is, from the trachea, and of 
the view of Mr. Simon, as afterwards expressed by the editors of Cuvier respecting the thyroid,- ‘ C’est la fausse 
branchie, branchiole des poissons.’ Indeed, from its relation to the air-tube during the early period of life, or 
in fish to the vertebral or hyoidal extremity of the gill, from its curious alternation with the supplementary gill 
of Broussonet, and from its structure (Kolliker), it may be not inaptly referred to as a pseudo-lung rather than 
as an associate with the thymus and the so-called ductless glands.” — Proceedings of the Boyal Society, vol. xvi. 
p. 185. 
