DEVELOPMENT OF THE MUSCLES AND SKIN. 397 



birth, the carpus is entirely cartilaginous and does not begin 

 to ossify until the second year ; and the same is true of the 

 tarsus, except the calcaneum and astragalus, which ossify just 

 before birth. The pisiform bone of the carpus is the last 

 to take on osseous transformation, this occurring at from the 

 twelfth to the fifteenth year. 1 As ossification progresses, the 

 deposits in the various ossific points gradually extend until 

 they reach the joints, which remain incrusted with the per- 

 manent articular cartilage. 



While the skeleton is being thus developed, the muscles 

 are formed from the outer layer of the intermediate blasto- 

 dermic membrane, and the visceral plates close over the tho- 

 rax and abdomen in front, leaving an opening for the um- 

 bilical cord. According to Burdach, the- various tissues of 

 the external parts, particularly the muscles, begin to be dis- 

 tinct at the end of the second month. 3 The deep layers of 

 the dorsal muscles are the first to be distinguished ; then, suc- 

 cessively, the long muscles of the neck, the anterior straight 

 muscles of the head, the straight and transverse muscles of 

 the abdomen, the muscles of the extremities, the superficial 

 muscles of the back, the oblique muscles of the abdomen, 

 and the muscles of the face. 



The skin appears at about the beginning of the second 

 month, when it is very delicate and transparent. At the end 

 of the second month, the epidermis may be distinguished. 

 The sebaceous follicles are developed at the third month ; 

 and, at about the fifth month, the surface is covered with 

 their secretion mixed with desquamated epithelium. This 

 cheesy substance constitutes the vernix caseosa. 3 At the 

 third month, the nails make their appearance, and the hairs 

 begin to grow at about the fifth month. 4 The sudoriparous 

 glands first appear at about the fifth month, by the formation 

 of flask-like processes of the true skin, which are gradually 



1 DALTON, Human Physiology, Philadelphia, 1871, p. 662. 



9 BDRDACH, Traite de physiologic^ Paris, 1838, tome iii., p. 416. 



8 See vol. iii., Secretion, p. 67. 4 BDRDACH, op. cit., p. 404, et seq. 



