94 



HUMAN ANATOMY. 



Nerves. — The periosteum contains a considerable number of nerves, the ma- 

 jority of which, however, are destined for the supply of the underlying osseous 

 tissue, since those distributed to the fibrous envelope of the bone are few. The 

 periosteal nerves follow the larger blood-vessels, in the walls of which they chiefiy 

 terminate. Medullary nerves accompany the corresponding blood-vessels through 

 the medullary canal, and within the marrow break up into fibrillae to be, probably, 

 distributed to the walls of the vascular branches along which they lie. Regarding 

 the ultimate endings and arrangement of the sensory fibres little is known ; in view 

 of the low degree of sensibility possessed by healthy bones and their periosteum, 

 the number of such nerves present in osseous structures must be very small. 



Fig. 124. 



DEVELOPMENT OF BONE. 



The bones composing the human skeleton, with few exceptions, are preceded 

 by masses of embryonal cartilage, which indicate, in a general way, the forms and 

 relations of the subsequent osseous segments, although many details of the model- 

 ling seen in the mature bones appear only after completed development and the pro- 

 longed exercise of the powerful modifying influences exerted by the action of the 

 attached muscles. Since the primary formation of such bones takes place within 

 the cartilage, the process is appropriately termed eyido chondral development. 



Certain other bones, notably those forming the vault of the skull and almost all 



those of the face, are not preceded by cartilage, but, on 

 the contrary, are produced within sheets of connective tis- 

 sue ; such bones are said, therefore, to arise by intra' 

 me^nbranous , development. It will be seen, however, that 

 the greater part of the bone formed by endochondral de- 

 velopment undergoes absorption, the spongy substance 

 within the ends of the long and the bodies of the irregu- 

 lar bones representing the persistent contribution of this 

 process of bone-production. Even in those cases in which 

 the intracartilaginous mode is conspicuous, as in the de- 

 velopment of the humerus, femur, and other long bones, 

 the important parts consisting of compact substance are the 

 product of the periosteal connective tissue, and hence ge- 

 netically resemble the intramembranous group. Although 

 both methods of bone-formation in many instances proceed 

 coincidently and are closely related, as a matter of con- 

 venience they may be described as independent processes. 

 Endochondral Bone Development. — The pri- 

 mary cartilage, formed by the proliferation and condensa- 

 tion of the elements of the young mesoblastic tissue, grad- 

 ually assumes the characteristics of embryonal cartilage, 

 which by the end of the second month of intra-uterine life 

 maps out the principal segments of the foetal cartilaginous 

 skeleton. These segments are invested by an immature 

 form of perichondrium, or primary periostejwi, from which 

 proceed the elements actively engaged in the production 

 of the osseous tissue. The primary periosteum consists of a 

 compact outer Jidrous and a looser inner osteogenetic layer; 

 the latter is rich in cells and delicate intercellular fibres. 

 The initial changes appear within the cartilage at points known as centres of 

 ossification^ which in the long bones are situated about the middle of the future shaft. 

 These early changes CFig. 125) involve both cells and matrix, which exhibit con- 

 spicuous increase in size and amount respectively. As a further consequence of this 

 activity, the cartilage-cells become larger and more vesicular, and encroach upon the 

 intervening matrix, in which deposition -of lime salts now takes place, as evidenced 

 by the gritty resistance offered to the knife when carried through such ossific centres. 

 On acquiring their maximum growth the cartilage-cells soon exhibit indications of 

 impaired vitality, as suggested by their shrinking protoplasm and degenerating 



Clarified human ftelus of about 

 three and oiie-half months, show- 

 ing the partially ossified skeleton. 

 Two-thirds natural size. 



