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THE SKELETON. 
By ARTHUR THOMSON. 
THE term skeleton (from the Greek, oxederos, dried) is applied to the parts which 
remain after the softer tissues of the body have been disintegrated or removed, 
and includes not only the bones, but also the cartilages and ligaments which bind 
them together. In the restricted sense of the word the skeleton denotes the 
osseous framework of the body. It is in this sense that it is generally employed in 
human anatomy. 
The skeleton serves to support the softer structures which are grouped around 
it, and also affords protection to many of the delicate organs which are lodged 
within its cavities. By the articulation of its several parts, its segments are con- 
verted into levers which constitute the passive portion of the locomotory system. 
Recent research has also proved that certain cells found in bone-marrow are 
intimately associated with the development and production of some of the 
corpuscles of the blood. 
Bone may be regarded as white fibrous tissue which, having become calcified, 
has undergone subsequent changes, so as to be converted into true osseous tissue. 
Most probably all bone is of membranous origin, but it may pass through a stage 
in which cartilage plays an important part in its development. In many instances 
the cartilage persists, and is not converted into bone, as in the case of the articular 
cartilage which clothes the joint surfaces, the nasal septum, the cartilages of the 
nose, and the cartilages of the ribs. A persistence of the membranous condition 
is met with in man in the case of the tentorium cerebelli, which in some groups 
of animals (Carnivora) is converted into a bony partition. 
Skeletal structures may be derived from each of the three layers of the 
trilaminar blastoderm. The exo-skeleton includes structures of ectodermal, and 
some of mesodermal origin in the shape of hair, nails, feathers, teeth, scales, armour- 
plates, ete., whilst the endo-skeleton, with which we are more particularly concerned, 
is largely derived from the mesoblastic tissue, but also includes the notochord, an 
entodermal structure which forms the primitive endo-skeleton, around which the 
axial skeleton is subsequently developed in the Vertebrata. The endo-skeleton is 
divisible into an axial portion, appertaining to the trunk and head, and an appen- 
dicular part associated with the limbs. It also includes the splanchnic skeleton, 
which comprises certain bones developed in the substance of some of the viscera, 
such as the os cordis and os penis of certain mammals. In man, perhaps, the 
cartilaginous framework of the trachea and bronchi may be referred to this 
system. The number of the bones of the skeleton of man varies according to age. 
Owing to a process of fusion taking place during growth, the number in the adult 
is less than the number in the child. 
The following table does not include the sesamoid bones which are frequently 
developed in tendons, the most constant ossicles of this description being those in 
relation to the metacarpo-phalangeal joint of the thumb, and metatarso-phalangeal 
joint of the great toe. 
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