64 



GENERAL ANATOMY. 



FIG. 36. Osteoblasts from the parietal bone of a human em- 

 bryo thirteen weeks old. (After Gegenbauer.) a. Bony septa with 

 the cells of the lacunae. 6. Layers of osteoblasts. c. The latter in 

 transition to bone-corpuscles. 



are contained. The cells at first lie upon the osteogenic fibres, so that they can 

 be removed by brushing the specimen with a hair-pencil, in order to render 



the fibres clear ; but they grad- 

 ually become involved in the 

 ossifying matrix, and form the 

 corpuscles of the future bone, 

 the spaces in which they are 

 enclosed constituting the la- 

 cunae. As the tissue increases 

 in thickness, vessels shoot into 

 it, grooving for themselves 

 spaces or channels, which be- 

 come the Haversian canals. 

 Thus, the intramembranous 

 and intracartilaginous processes 

 of ossification are similar in 

 their more essential features. 



The number of ossific cen- 

 tres is different in different 

 bones. In most of the short 

 bones ossification commences 

 by a single point in the centre, 

 and proceeds toward the cir- 

 cumference. In the long bones there is a central point of ossification for the 

 shaft or diaphysis; and one or more for each extremity, the epiphysis. That 

 for the shaft is the first to appear. The union of the epiphyses with the shaft 

 takes place in the reverse order to that in which their ossification began, and 

 appears to be regulated by the direction of the nutrient artery of the bone. Thus, 

 the nutrient arteries of the bones of the arm and fore-arm are directed toward the 

 elbow, and the epiphyses of the bones forming this joint become united to the 

 shaft before those at the opposite extremity. In the lower limb, on the other 

 hand, the nutrient arteries pass in a direction from the knee : that is, upward in 

 the femur, downward in the tibia and fibula ; and in them it is observed that the 

 upper epiphysis of the femur, and the lower epiphysis of the tibia and fibula, 

 become first united to the shaft. 



Where there is only one epiphysis, the medullary artery is directed toward 

 that end of the bone where there is no additional centre, as toward the acromial 

 end of the clavicle, toward the distal end of the metacarpal bone of the thumb 

 and great toe, and toward the proximal end of the other metacarpal and meta- 

 tarsal bones. 



Besides these epiphyses for the articular ends, there are others for projecting 

 parts or processes, which are formed separately from the bulk of the bone. For 

 an account of these the reader must be referred to the description of the individual 

 bones in the sequel. 



A knowledge of the exact periods when the epiphyses become joined to the 

 shaft is often of great importance in medico-legal inquiries. It also aids the sur- 

 geon in the diagnosis of many of the injuries to which the joints are liable; for it 

 not infrequently happens that, on the application of severe force to a joint, the 

 epiphysis becomes separated from the shaft, and such injuries may be mistaken for 

 fracture or dislocation. 



MUSCULAR TISSUE. 



The muscles are formed of bundles of reddish fibres, endowed with the property 

 of contractility. Two kinds of muscular tissue are found in the animal body viz. 

 that of voluntary or animal life, and that of involuntary or organic life. The 

 muscles of animal life (striped muscles) are capable of being put in action and 

 controlled by the will. They are composed of bundles of fibres enclosed in a 



