642 EMBRYOLOGY 



The further growth of the skeletal element thus composed of two 

 tissues proceeds in two ways : first by growth of the cartilage, and 

 secondly by increase of bony substance. 



The cartilaginous tissue increases at both ends of the skeletal 

 piece and contributes to the increase of the latter both in length and 

 thickness. In the middle, on the contrary, where it is enveloped in 

 a bony cylinder, it ceases to grow. Here there is a continual ad- 

 dition of new bony lamellae upon those already formed; they are 

 produced by the original perichondrium, or, as one may now more 

 properly say, by the periosteum. 



In this process the successive lamellae extend farther and farther 

 toward the two ends of the skeletal piece; new portions of the 

 cartilage are being continually ensheathed in bone and restricted in 

 their growth. 



The periosteal bony sheath assumes in consequence the form of 

 two funnels united at their apices. 



The cartilage which fills up the funnels early undergoes a gradual 

 metamorphosis and degeneration. From the osseous sheath there 

 grow into it connective-tissue strands with blood-vessels, which 

 dissolve the matrix and produce larger and smaller marrow-cavities. 

 Then, by the secretion of osseous tissue at the surface of the 

 persisting remnants of cartilage, there is developed a spongy bone- 

 substance, which fills up the funnel-shaped cavities of the compact 

 bony mantle produced by the periosteum. The spongy bone is, 

 however, only an evanescent structure. It in turn is gradually 

 dissolved, beginning at the middle of the skeletal element, and its 

 place is occupied by a very vascular marrow. In this way there 

 arises in the originally quite compact cartilaginous fundament the 

 large central medullary cavity of the long bones. 



During these processes the two ends still remain cartilaginous, and 

 serve for a long time by their growth to increase the length of the 

 skeletal element. They are designated as the two epiphyses, in 

 distinction from the middle piece, which is the first to ossify, and 

 which has received the name diaphysis. The latter increases in size 

 at the expense of the epiphysial cartilages, for the endochondrai 

 process of ossification progresses, with a very distinct line of ossifica- 

 tion, toward both ends. 



A new complication in the development of the tubular (long) 

 bones arises either a short time before or in the first years after 

 birth. There are then developed in the middle of each epiphysis 

 special centres of ossification, the so-called epiphysial nuclei; there 



