THE BRAXCHIAL REGION 



71 



vomero-nasal organs of Jacobson, which open below, close to the junction 

 of the premaxillary and maxillary bones. 



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 



e 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 w r hich 

 contribute to their formation the 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 furrows the 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, 



Hyoid 

 Mandibular arch 



Maxillary 'process 

 Eye 



Auricula 



Fore-limb 



Hind-limb 



Hind-limb 



IG. 52. Human embryo from thirty-one to thirty- 

 four days. (His.) 



Umbilical cord 



FIG. 53. Embryo of about six weeks. (His. 





and each presents two surfaces and two borders. Of the surfaces, one the future 

 flexor surface of the limb is 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 

 b, inward and backward. As a consequence of this rotation the preaxial (radial) 



