100 THE STRUCTURE OF MAN 
an anterior tract. The former arises from the pelvis, and is 
inserted anteriorly, as a rule, on a level with the fifth rib; the 
latter 1s represented by the ventral cervical muscles, viz. the 
sterno-hyoid and sterno-thyroid, which here and there bear 
inscriptiones tendineze indicative of their former segmentation. 
To these must be added the almost constant omo-hyoid, which 
is provided with an inscriptio, and the thyro-hyoid. Farther 
forward these are joined by the hyo-glossus, genio-hyoid and 
genio-glossus, which belong to the same metameric series.’ 
In the lower Primates the rectus abdominis muscle still 
reaches to near the first rib, and thus recalls the connection 
with the cervical musculature mentioned above, which was first 
lost in the Reptiles. Even in Man it may sometimes run beyond 
the fifth rib and, under cover of the pectoralis major, pass as far 
up as the second. This is a striking case of atavism. 
In the higher Primates the thoracic head of this muscle 
shifts back to the lower ribs, and this shifting towards the 
abdominal region is accompanied by an advancing loss of 
segmentation in both the Anthropoids and Man.” But even 
where this is most marked the muscle has not quite lost its 
thoracic character. 
This retreat of the rectus muscle is intimately connected 
with the development of the great adductor of the fore-limb (the 
pectoralis major), since it 1s only when the upper parts of the 
rectus disappear that the muscular bundle forming the pectoralis 
major—and, indeed, that forming the pectoralis minor as well— 
is able to take possession of the firm anterior thoracic surface 
furnished by the ribs. Where, as in the lower Apes, the anterior 
end of the rectus muscle covers the thorax as far as the lateral 
edges of the sternum, a primitive condition being thus retained, 
those fasciculi of the pectoral muscles which arise from the 
skeleton come simply from the sternum. “ We here have a con- 
flict at close quarters between different parts of the organism ” 
(Ruge).° 
In connection with his studies of the abdominal musculature, 
1 (Cf. Albrecht. Beitrag z. Morphologie des M. omo-hyoides wu. d. ventr. imneren 
Interbranchialmusculatur t. d. Reihe d. Wirbelthiere.—Inaug. Diss., Kiel., 1876.] 
> In many cases the muscle withdraws in a distal direction even farther than 
the fifth rib, and derives its anterior (uppermost) slip from the sixth. A primitive 
slip from the eighth rib may also be retained (Ruge). 
3 Where, as a rare anomaly, the rectus abdominis is double on one or on both 
sides, a very low condition is indicated, this arrangement being typical in Amphibia 
and Saurians. 
