4 2 NORMAL HISTOLOGY. 



Yellow Marrow. After early childhood, the red marrow, which pre- 

 viously fills all the medullary spaces, begins to suffer invasion by fat-cells 

 and conversion into adipose tissue. This change, which results from the 

 substitution of fat-cells for the distinctive marrow components, affects the 

 medullary tissue within the bones of the extremities, except within the upper 

 ends of the humerus and femur. Examined in section, the yellow marrow 

 resembles ordinary adipose tissue, consisting chiefly of the large compressed 

 spherical fat-cells supported by a reticulum of connective tissue. The cells 

 belonging to the latter, occasional plasma cells and myelocytes are the 

 usual elements encountered. In advanced age and during starvation, the 

 customary consistence and yellow color give place to a mucoid condition 

 and reddish tint, the fat-cells of such gelatinous marrow, as it is termed, 

 losing much of their oily contents. 



Blood-Vessels of Bones. The generous blood-supply of bones is 

 arranged as two sets, the periosteal and the medullary. The former consti- 

 tutes a network within the periosteum and supplies twigs, which enter the 

 subjacent compact bone through the Volkmann canals and communicate 

 with the branches from the medullary system. The medullary artery is 

 often, as in the case of the long bones, a vessel of considerable size, wl.ich, 

 accompanied by the companion veins, traverses the oblique nutrient canal 

 to gain the centre of the marrow. On reaching this position the medullary 

 artery usually divides into ascending and descending branches, from which 

 twigs radiate towards the periphery of the marrow cavity. The twigs termi- 

 nate in arterial capillaries, which expand rather abruptly into larger venous 

 capillaries, in consequence of this arrangement the rapidity of the blood- 

 stream becoming diminished in its course through the marrow. Within the 

 red marrow, the venous capillaries possess an imperfect endothelial lining, 

 thereby affording an opportunity for the newly formed blood-cells, the eryth- 

 rocytes and the leucocytes, to gain entrance into the circulation. After thus 

 coming into close relations with the marrow-tissue, the blood is collected by 

 the capillaries which form veins destitute of valves. In addition to the com- 

 panion veins which accompany the medullary artery through the nutrient 

 canal, in many instances the larger veins pursue an independent course and 

 emerge from the cancellous tissue by means of special canals piercing the 

 compact substance. 



Definite lymphatics are found only within the outer layer of the perios- 

 teum, although the system of intercommunicating spaces within the bone, the 

 lacunae, and canaliculi, stands in close relation with the larger lymph-channels. 



The nerves supplying the bones include both medullated and nonmed- 

 ullated fibres. The latter, distributed partly to the periosteum and partly 

 within the bone, are chiefly sympathetic fibres destined for the control of the 

 involuntary muscle within the walls of the blood-vessels. The medullated 

 sensory fibres are few, some being connected with special endings of the 

 lamellar type within the periosteum. 



DEVELOPMENT OF BONE. 



With the exception of certain parts of the skull the vault and nearly 

 all of the face the bones of the human skeleton are preceded by solid 

 masses of embryonal hyaline cartilage. Since the primary development of 

 such bones takes place within the cartilage, they are known as cartilage 

 bones and the mode of formation is termed endochondral development. The 

 bones not preceded by cartilage are produced within sheets of connective 



