BONES OF THE TRUNK. 15 



Marroio is contained in a fine vascular membrane, lining the me- 

 dullary canals and cells of bones called the Internal Periosteum. 

 It resembles fat, but consists of finer granules ; in consumption, 

 dropsy, &c., it is absorbed and its place is supplied with serum. 



Formatiott and growth of bones. — There are three stages of ossifi- 

 cation in the embryo. The first is the mucous or pulpy, which con- 

 tinues for one month ; the second is the cartilaginous, and the third 

 the osseous, commencing at the third month. The colour of the 

 cartilage deepens and then a vessel conveys red blood to a centra! 

 point, which first receives calcareous particles and is called punctuin 

 ossijicationis. Bones increase in length by continued deposit at 

 their extremities between the Diaphysis and the Epiphyses, as is 

 proved by Hunter's experiment of placing two shot in the tibia of a 

 young pig ; 'after the animal had reached full size, the shot were 

 found at their original distance from each other. Bones increase in 

 thickness by external deposit and by secretion from the periosteum, 

 which is proved by disease and the experiment of feeding a young 

 pig with food coloured with madder. Various laminte of white and 

 coloured bone can be produced by suspending and resuming this 

 mode of feeding. While deposit is taking place upon the surface of 

 a bone, absorption is going on internally ; this is proved by Duhamel's 

 experiment of surrounding a long bone of a young animal with a 

 metallic ring; after the animal is fully grown, the ring was found 

 in the medullary canal, which increases in size by this constant de- 

 posit and absorption. 



Callus. — When bones are fractured, the process of reparation 

 resembles that of the formation of a bone. Blood is effused, lymph 

 is thrown out, causing a swelling; the blood is absorbed, lymph 

 coagulates and ossifies in the form of a ring, surrounding the seat of 

 fracture, and also in the form of a pin in the medullary canal. The 

 fractured extremities begin to unite and coalesce, after which the 

 superfluous ring and pin of bone which acted as splints are absorbed. 

 Whilst this process is going on, it is important that the parts should 

 be maintained at rest, else the process of ossification is arrested, and 

 a false joint results. 



BONES OF THE TRUNK. 



The trunk consists of the Spine, Pelvis, and Thorax. 



SPINE. ^ 



It extends from the head to the coccyx at the posterior part of the 

 trunk, having several curvatures in its extent. In the neck it is 

 convex anteriorly, in the thorax concave, in the loins convex, and 

 in the pelvis concave. It contains an osseous canal for the spinal 

 marrow, and is formed of 28 or 29 separate bones called vertebrae, 



