FORIvIATIOX OF THE JOINTS. 7J5 



formative cells in the outer or superficial part of the protovertel)raI 

 masses. Some difference of o]iinion exists, however, among embryolo- 

 gists, as to how far the hypaxial (hyposkeletal of Huxlc}') as well as 

 the epaxial muscles, proceed from this source alone, or whether only 

 the latter are traceable to the muscular plate formed by the proto- 

 vertebral differentiation, and the hyj^axial may be supposed to proceed 

 from a deeper source. 



Eeccnt observations seem to show that a downward extension of the 

 mesoblast from the protovertebrge may also give rise to the hypaxial 

 muscles. 



Being developed from the segmented protovertebral elements, the 

 muscular plate shows at first the same division into segments, which 

 ai-e separated for a time by intermuscular septa (myotomes) as occurs 

 during life in a considerable number of them in fishes and amphibia. 



The formation of the longer muscles of the trunk proceeds from the 

 disappearance of the septa, and the longitudinal union of the fasciculi 

 of successive myotomes. In the trunk the direction of these remains 

 for the most part chiefly longitudinal, but those connected with the 

 limb-girdles change their direction with the development of the limb. 



The formation of the muscles of the limbs themselves has not been 

 traced in detail. The greater number of these muscles appear rather 

 to arise independently in the blastodermic tissue of the limb-bud, than 

 to be prolonged from the sheets of trunk-muscles (Kiilliker). 



The facial muscles and the platysma, belong to the subcutaneous 

 system, and are developed along with the skin. 



The diaphragm is at first wanting. It arises soon after the forma- 

 tion of the lungs, fi'om two parts which spring from above and the 

 sides, and which divide the pleural and peritoneal cavities, which were 

 jDreviously in one, from each other. 



The muscles begin to be formed in the human embryo in the sixth 

 and seventh week. 



Formation of the Joints. — With regard to the formation of the 

 joints, very little is known. It would appear that the cavities of the 

 synovial joints are not yet formed at the time when chondrification 

 has taken place in the matrix of the bones. It is therefore by a 

 secondary process of solution of continuity that these cavities are pro- 

 duced. The articular cartilages remain as the coverings of the opposed 

 surfaces of the bones, and the various ligamentous and other parts 

 belonging to the joints arise by processes of textural differentiation 

 which it is unnecessary to particularise here. 



Distinction of Bones according to their Cartilaginous or Membranous 



Origin. — There is here appended for the assistance of the reader a note of the 

 distinction as regards their origin from cartilage or fibrous membrane of the 

 .several pennanent bones of the skeleton. 



1. Boiu'.s- (irisiiif/from Cartilage: — 



fl, III the Head. 



Basi-occipital. ex-occipital, and part of the supra-occipital or squama occipitis. 



The whole sphenoid except the cornua sphenoidalia. 



The p.etro-mastoid or periotic portion of the temporal bone. 



The mes-ethmoid and ethmo-turbinal. 



The pterygo-palatine. 



The malleus (quadi-ate of animals) with Meckel's cartilage. 



The incus and stapes, with the stylo-hyoid. 



The thyro-hyoid. 



