OSTEOLOGY. 



THE SKELETON. 



By ARTHUR THOMSON, F.RC.S. 



Professor of Anatomy, University of Oxford. 



THE term skeleton (from the Greek, o-KeAeros, 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. 

 Kecent research has also proved that certain cells found in bone -maf row 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 ( Garni vora) 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, etc., whilst the endo-skeleton, with which we are more particularly concerned, 

 is largely derived from the mesodermal 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 the metatarso-phalangeal joint of the great toe. 



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