SOME OF THE SUBAXIAL ARCHES IN MAN. 
125 
clavicle, which is also connected with the first rib, the anterior portion of the seventh arch 
is brought from an oblique into a line nearly horizontal (in Man), and at this period 
the portion of membrane above it, which at first lies close along the upper border of the 
clavicle, becomes detached from the outer half of this border, or is united only by a thin 
layer of tissue. The tissue which remains connected with the inner half, or thereabouts, 
of the clavicle forms the sterno-mastoid muscle. 
The inner extremity (Plate I. fig. 7, a) of the rod of cartilage (which is the scapula) 
being connected with the outer extremity of the clavicle is brought into the same hori- 
zontal line with that bone and forms the acromion. The coracoid extremity of the 
scapula (Plate I. fig. 7 and following, co ) becomes more curved upon the acromial, to 
the level of which it eventually reaches (chiefly by the straightening in a downward direc- 
tion of the acromion), and thus the entire scapula forms a slightly spiral curve, and the 
root of the coracoid portion becomes attached to the clavicle by a strong fibrous band. 
In this spiral curving of the rod of cartilage which forms the scapula the upper surface 
of the coracoid corresponds with the upper surface, its internal surface with the anterior, 
and its external with the posterior border of the acromion process. By the same curving 
of this portion of the arch the lower portion of the posterior border of the membrane 
ascending to the occipital region is bent or folded inwards ; and this seems to explain the 
connexions acquired by the levator anguli scapuke muscle in the cervical region, and its 
insertion into the end of the plate of the scapula which grows from the root of the cora- 
coid process. 
The growth of the plates of the scapula takes place from the posterior border of the 
rod of cartilage, the form of the bone being determined by the curve of the primitive 
rod. Following the posterior border the acromion plate is seen (fig. 8, a ) growing 
backwards, whilst from the remainder of the rod to the root of the coracoid the larger 
plate bounded by the glenoid border is developed. Scarcely any growth has yet taken 
place from the coracoid (fig. 8, co), but in a foetus 1*2 (fig. 9, co) it is beginning to 
throw out its plate, and in a foetus 1*8 (fig. 10, co. p) this plate is being rapidly com- 
pleted. The scapula is thus built up in the three-sided prismatic form referred to by 
Professor Flower*. 
Later, comparatively, in the formation of the scapula (fcetus L8, fig. 10) the glenoid 
cavity grows out from the rest of the cartilage, and thus leaves a considerable notch be- 
tween it and the acromion, and the scapula now acquires its permanent characters. 
Up to this point no ossification is observable, nor have I recognized any segmentation 
of the primitive rod. When ossification commences it begins, as shown by Mr. Parker, 
in the central portion of the rod, as is the case with the clavicle, and, as with the 
clavicle, it leaves two cartilage ends, which are the acromion and the coracoid process. 
* Osteology of the Mammalia, p. 334. 
MDCCCLXXI. 
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