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 lacuna?, 

 connected together by delicate channels or canaliculi, which radiate 

 in all directions from the sides of the lacuna?. 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. 1 



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 

 Avithin 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 Ost6ographie of De Blainville, 1835-54 ; the section 

 devoted to the subject in Bronn's Klasscn und Ordnungen des Thicr-Rcichs, by 

 Giebel, 1874-79 ; and An Introduction to the Osteology of the Mammalia, by 

 W. II. Flower, 3d ed., 1885. 



