THE OSSEOUS SYSTEM. 267 



termed subst. ceUuJaris, and when smaller, suhst. reticidaris. The latter, 

 in some situations where the cavities are smaller, and the osseous parti- 

 tions stronger, approaches in character the compact substance, although 

 it does not actually become such ; and in others it passes without any 

 defined limit, into compact tissue. This does not, however, prove that 

 the two substances are identical, but as we learn from observation of 

 their development, depends simply upon the circumstance that the 

 spongy substance very frequently arises in a partial expansion of the 

 compact. The share taken by the two substances in the formation of 

 the different bones, and parts of bones, varies very considerably. It is 

 only in a few situations that the compact substance is met with by itself 

 without vascular canals — as in the lamina ipa-pyracea of the ethmoid 

 bone, some portions of the lachrymal and palate bones, &c. It occurs 

 more frequently, however, with vascular canals, and without spongy 

 substance as in many individuals in the thinnest portion of the scapula, 

 ilium, acetabulum, cranial bones {ala magna, parva of the sphenoid, the 

 orbital process of the frontal bone, &c.) Spongy substance with a thin 

 compact cortex, without vascular canals, exists in the auditory bones, on 

 the surfaces covered with cartilage of all bones, probably also in the 

 smaller spongy bones. In all other cases, and consequently in most 

 situations, the two substances are conjoined, but in such a way, that 

 sometimes the spongy substance predominates (spongy bones and parts 

 of bones), as in the vertebra', carpal and tarsal bones ; sometimes the 

 compact, as in the diaphyses of the long bones ; or the two are in equal 

 proportions, as in the fiat bones. 



§ 89. Intimate Structure of the Osseous Tissue. — The osseous tissue 

 consists of a dense, for the most part indistinctly lamellar fundamental 

 substance or matrix, penetrated by vascular canals and numerous minute 

 microscopic spaces — the bone-cells, or lacuna' (bone-corpuscles of authors), 

 having very minute hollow processes, the bo7ie-canaliculi. 



The vascular canals of the bones, or the Haversian canals {canalieuli 

 medullares), are minute tubules, having an average diameter of 0-01- 

 0-05 of a line, and in the extremes one varying from O-OO-l to 0-18 of a 

 line, and which, except in the thinner parts of the facial bones, as above 

 mentioned, exist universally in the compact substance, forming in it a 

 wide network similar to that of the capillaries. In the long bones, and 

 also in the ribs, clavicle, pubis, ischium, and lower jaw, they run chiefly 

 in a direction parallel to the long axis of the bone, and, as shown in 

 longitudinal sections, either parallel to the surface or perpendicular to 

 it, at distances varying from 0-06 to 0-14 of a line apart. They are 

 connected by transverse or oblique branches, which run in the direction 

 both of the radius and of the tangents of a transverse section of the 

 bone. Consequently, under a low magnifying power, in longitudinal 



