THE STRUCTURE OF BONE. 191 



§ 563. The capacity of a prepared skull may be obtained by filling it through the 

 Fm. magnum with sand or fine shot, and then pouring the material into a graduate glass. 

 If the material used in determining the capacity is fine enough to pass through the fora- 

 mina, they must be plugged in some way. 



The weight of the brain may be obtained approximately by reckoning the cvbic centi- 

 meters of capacity as grmns and adding 4 per cent. (Wyman, 7(>). Thus, if a cat's skull 

 has a capacity of 25 cc., the brain of the same cat would weigh approximately 26 grams. 

 Wyman's statement refers only to the human brain, but presumably the specific gravity of 

 the cat's brain is nearly or quite identical with that of man. 



§ 565. Obvious Structure of Bone. — In life the surface is covered with a dense layer 

 of connective tissue, the periosteum. Entad of this periosteum is a layer of compact bone. 

 The intermediate part of all bones is, however, more or less loose in structure, something 

 like a sponge, hence it is called spongy or cancellated bone. This is especially abundant 

 toward the end of long bones and in the vertebral centra. 



§ 566. Microscopic Structure. — A solid mass containing : A. Haversian canals, cyl- 

 indrical channels shown as circles in cross section, as cylinders in longitudinal sections 

 of long bones. These canals contain the blood vessels, as may be demonstrated by exam- 

 ining a finely injected cat's scapula (see Frey, A). They anastomose freely, and open 

 either upon the ectal surface or within the medullary canal. 



B. Lacunce and canalicuU. These are the spaces occupied by the protoplasmic bone 

 cells or corpuscles and their prolongations. They appear in outline like irregularly 

 fusiform connective tissue corpuscles with many fine prolongations or branches. These 

 branches anastomose with the branches of neighboring lacunae, and sometimes open into 

 an Haversian canal. 



In transections of long bones the solid matter and lacunae are seen to be arranged in 

 more or less concentric lamellfe around the Haversian canals. 



