34 GENERAL ANATOMICAL CHARACTERS 
histological structure and chemical composition, being formed 
mainly of a gelatinous basis, strongly impregnated with salts of 
calcium, chiefly phosphate, and disposed in a definite manner, con- 
taining numerous minute nucleated spaces or cavities called lacune, 
connected together by delicate channels or canaliculi, which radiate 
in all directions from the sides of the lacunz. Parts composed of 
bone are, next to the teeth, the most imperishable of all the organs 
of the body, often retaining their exact form and internal structure 
for ages after every trace of all other portions of the organisation 
has completely disappeared, and thus, in the case of extinct animals, 
affording the only means of attaining a knowledge of their characters 
and affinities.? 
In the Armadillos and their extinct allies alone is there an. 
ossified exoskeleton, or bony covering developed in the skin. In , 
all other mammals the skeleton is completely internal. It may be 
described as consisting of an axial portion belonging to the head 
and trunk, and an appendicular portion belonging to the limbs. 
There are also certain bones called splanchnic, being developed 
within the substance of some of the viscera. Such are the os cordis 
and os penis found in some mammals. 
It is characteristic of all the larger bones of the mammalia that 
their ossification takes its origin from several distinct centres. One 
near the middle of the bone, and spreading throughout its greater 
portion, constitutes the diaphysis, or “shaft,” in the case of the long 
bones. Others near the extremities, or in projecting parts, form 
the epiphyses, which remain distinct during growth, but ultimately 
coalesce with the rest of the bone. 
Axial skeleton.—The axial skeleton consists of the skull, the 
vertebral column (prolonged at the posterior extremity into the 
tail), the sternum, and the ribs. 
Skull.—In the skull of adult mammals, all the bones, except the 
lower jaw, the auditory ossicles, and the bones of the hyoid arch, 
are immovably articulated together, their edges being in close con- 
tact, and often interlocking by means of fine denticulations project- 
ing from one bone and fitting into corresponding depressions of the 
other ; they are also held together by the investing periosteum, or 
fibrous membrane, which passes directly from one to the other, 
and permits no motion, beyond perhaps a slight yielding to external 
pressure. In old animals there is a great tendency for the different 
bones to become actually united by the extension of ossification 
from one to the other, with consequent obliteration of the sutures. 
1 See for the principal modifications of the skeleton of the class, the large 
and beautifully illustrated Ostéographie of De Blainville, 1835-54; the section 
devoted to the subject in Bronn’s Klassen wnd Ordnungen des Thier-Reichs, by 
Giebel, 1874-79; and An Introduction to the Osteology of the Mammalia, by 
W. H. Flower, 3d ed., 1885. 
