88 



STUDIES IN ADVANCED PHYSIOLOGY. 



THE MINUTE STRUCTURE OF BONES. 



Little need be said concerning the gross structure of 

 bones. A mere glance will show one the difference between 

 the long bones, such as the femur or hu~ 

 merus, the short bones, such as those of 

 the tarsus and carpus, the tabular bones, 

 like the parietal bones of the skull, and the 

 irregular bones, which do not seem to fit 

 in any of the preceding classes. If, fur- 

 ther, a long bone, such as the humerus, 

 say, be examined, it is easily seen to con- 

 sist of a long shaft made up of hard, dense, 

 apparently homogeneous bone, with some- 

 what expanded ends, intended for articu- 

 lating surfaces. Numerous little holes are 

 visible, through which blood-vessels and 

 nerves enter the bone. If such a bone be 

 sawed in two a large, empty space appears 

 through the shaft, known as the medullary 

 cavity. It is filled in life with a yellowish 

 substance consisting mostly of fat, called 

 the yellow marrow. At the end the bone 

 is seen to be cancellated or spongy. In 

 this cancellated bone is found the red mar- 

 row, the seat of the formation of the red 

 corpuscles of the blood. Further than this 

 nothing can be made out as to the struc- 



If a tabular 

 bone such as the parietal be examined, 



Fig. 39. THE GROSS AN- A .-1,1 -. i 



ATOMY OP A LONG ture with the unaided eye. 



BONE. (Humerus.) 



numerous little openings into it for the 

 entrance and exit of blood-vessels 



a, medullary 

 containing the marrow; 

 I, shaft of hard bone; c, 

 spongy or cancellated 



are 



bone; d, terminal card- a g a | n v i s ibl e , while if it be broken it shows 



that it is made up of two plates, the denser 



bone on the outside and an intervening layer of spongy 



bone called diploe. The structure of the parietal bone is 



