THE YELLOW BONE-MARROW. 93 



blasts, are directly converted into the mature, non-nucleated red blood-disks on the 

 disappearance of their nucleus. In addition to a larger amount of haemoglobin in their 

 cytoplasm, the erythroblasts differ from the normoblasts in the possession of a deeply 

 staining nucleus, in which the chromatin no longer appears as a, reticulum. 



It is usual to find isolated groups of fat-cells distributed within the red marrow, 

 although the amount of adipose tissue is very meagre in localities farthest removed 

 from the medulla of the long bones. The varieties of leucocytes usually seen in the . 

 blood are also encountered within the red marrow in consequence of the intimate 

 relations between the latter tissue and the blood-stream conveyed by the medullary 

 capillaries. 



Yellow Marrow. — Since the appearance of the yellow marrow is due to the 

 preponderating accumulation of fat-cells which have replaced the typical elements 

 of the marrow contained within the shaft of certain bones, the formation of this 

 variety is secondary and must be regarded as a regression. 



Examined in section, yellow marrow resembles ordinary adipose tissue, since 

 it consists chiefly of the large oval fat-cells supported by a delicate reticulum of 

 connective tissue. In localities in which the latter exists in considerable quantity, 

 numerous lymphoid cells represent the remaining elements of the originally typical 

 marrow-tissue. After prolonged fasting the yellow marrow loses much of its oily 

 material and becomes converted into a gelatinous substance containing compara- 

 tively few fat-cells ; upon the re-establishment of normal nutrition this tissue may 

 again assume the usual appearance of yellow marrow. 



Blood-Vessels.-^The generous blood-supply of bones is arranged as two sets 

 of vessels, \^^ periosteal and the medullary. The former constitutes an external 

 net-work within the periosteum, from which, on the one hand, minute twigs enter 

 the subjacent compact substance through channels ( Volkmann' s canals') communi- 

 cating with the Haversian canals, within which they anastomose with the branches 

 derived from the medullary system ; additional vessels, on the other hand, pass to 

 the cancellated tissue occupying the ends of the long bones. 



The medullary artery is often, as in the case of the long bones, a vessel of con- 

 siderable size, which, accompanied by companion veins, traverses the compact sub- 

 stance through the obliquely directed 7nedullary canal to gain the central part of the 

 marrow. On reaching this position the medullary artery usually divides into ascend- 

 ing and descending branches, from which radiating twigs pass towards the periphery. 

 The latter terminate in relatively narrow arterial capillaries, which, in turn, expand 

 somewhat abruptly into the larger venous capillaries. Such arrangement results in 

 diminished rapidity of the blood-stream, the blood slowly passing through the net- 

 work formed by the venous capillaries. The latter vessels, within the red marrow, 

 possess thin walls and an imperfect endothelial lining in consequence of which the 

 blood comes into close relation with the elements of the medullary tissue. During 

 its sluggish course within the blood-spaces of the red marrow, the blood takes up the 

 newly formed red cells, which thus gain entrance into the circulation to replace the 

 effete corpuscles which are continually undergoing destruction within the spleen. It 

 is probable that leucocytes also originate in the bone-marrow. 



After thus coming into intimate relations with the marrow-tissue, the blood is 

 collected by capillaries which form small veins. In addition to the companion veins 

 accompanying the nutrient artery along the medullary canal, in many instances the 

 larger veins pursue a course independent of the arteries and emerge from the can- 

 cellous tissue by means of the canals piercing the compact substance at the ends of 

 the bones. Although destitute of valves within the medulla, the veins possess an 

 unusual number of such folds immediately after escaping from the bone. 



Lymphatics. — The definite lymphatic channels of the bones are principally 

 associated with the blood-vessels of the periosteum and the marrow as perivascular 

 channels, although it is probable that lymphatic spaces exist within the deeper layers 

 of the periosteum, in close relation to the osseous tissue. The perivascular lym- 

 phatics follow the blood-vessels into the Haversian canals, where, as well as on other 

 surfaces upon which the canaliculi open, the system of intercommunicating juice- 

 channels represented by the lacunae and the canaliculi is closely related with the 

 lymphatic trunks. 



