Henry Gray (18251861). Anatomy of the Human Body. 1918.
the vomero-nasal organs of Jacobson, which open below, close to the junction of the premaxillary and maxillary bones.
FIG. 51 Frontal section of nasal cavities of a human embryo 28 mm. long. (Kollmann.) (See enlarged image)
The Limbs.The limbs begin to make their appearance in the third week as small elevations or buds at the side of the trunk (Fig. 52). Prolongations from the muscle- and cutis-plates of several primitive segments extend into each bud, and carry with them the anterior divisions of the corresponding spinal nerves. The nerves supplying the limbs indicate the number of primitive segments which contribute to their formationthe upper limb being derived from seven, viz., fourth cervical to second thoracic inclusive, and the lower limb from ten, viz., twelfth thoracic to fourth sacral inclusive. The axial part of the mesoderm of the limb-bud becomes condensed and converted into its cartilaginous skeleton, and by the ossification of this the bones of the limbs are formed. By the sixth week the three chief divisions of the limbs are marked off by furrowsthe upper into arm, forearm, and hand; the lower into thigh, leg, and foot (Fig. 53). The limbs are at first directed backward nearly parallel to the long axis of the trunk, and each presents two surfaces and two borders. Of the surfaces, onethe future flexor surface of the limbis directed ventrally; the other, the extensor surface, dorsally; one border, the preaxial, looks forward toward the cephalic end of the embryo, and the other, the postaxial, backward toward the caudal end. The lateral epicondyle of the humerus, the radius, and the thumb lie along the preaxial border of the upper limb; and the medial epicondyle of the femur, the tibia, and the great toe along the corresponding border of the lower limb. The preaxial part is derived from the anterior segments, the postaxial from the posterior segments of the limb-bud; and this explains, to a large extent, the innervation of the adult limb, the nerves of the more anterior segments being distributed along the preaxial (radial or tibial), and those of the more posterior along the postaxial (ulnar or fibular) border of the limb. The limbs next undergo a rotation or torsion through an angle of 90° around their long axes the rotation being effected almost entirely at the limb girdles. In the upper limb the rotation is outward and forward; in the lower limb, inward and backward. As a consequence of this rotation the preaxial (radial)