18 STRUCTURE OF THE BONES. 



blows on the head, the brain, though much protected from 

 external injury by the arched form of the cranium, has an 

 additional security from the interposition of the diploe, which 

 weakens the force of the blow. 



In all the bones there are canals, independent of the cellular 

 structure, which penetrate to a greater or less extent between 

 the lamina, and go in various directions, some longitudinal, 

 others oblique and transverse. These canals transmit the blood- 

 vessels, and were first pointed out with exactitude by Clopton 

 Havers, an English anatomist. But he assigned a wrong 

 application to them, as he believed that the marrow ran through 

 them, in order to make the bones supple, and to unite their 

 lamina more strongly. S. B. Albinus corrected the mistake, by 

 demonstrating that they were filled with blood-vessels. These 

 canals in a vertebra are particularly large, and open on the 

 posterior face of its body, by one or two large foramina. In 

 the cranium they are remarkably well seen ; but their discovery 

 is of more modern date. M. Portal says, that in the bones 

 each kind of vessel has a particular canal for itself alone ; those 

 of the arteries are therefore to be readily distinguished from 

 such as belong to the veins and to the nerves ; and this takes 

 place both in the large and in the small canals. Occasionally 

 the vessels dip into a common canal, but if any one will take 

 the trouble to follow them, he will find them ultimately 

 separating from each other.] 



— The canals for the transmission of blood-vessels, which 

 exist in abundance in the compact bony tissue, cannot be well 

 seen in the healthy state, except by the aid of the microscope. 

 With the aid of this instrument they may be seen in great 

 numbers, running in a longitudinal direction, opening in its 

 internal or medullary cavity, so as to maintain a free commu- 

 nication between the vessels on the exterior, and those in the 

 cavity of the bone. 



— When cut in surgical operations, blood issues from the 

 compact substance, which is also susceptible of inflammation 

 and its consequences like other vascular parts. In inflammation, 



