ORGAN AND BODY SYSTEMS 511 



which are especially sensitive. The dentine structures are destroyed, 

 but the enamel is not much changed. Smaller doses bring about pulp 

 atrophy. The further development of the teeth is retarded. In the 

 dog, growth of the teeth may be stopped altogether and they may fall out. 

 In such a case the periodontoid tissues, alveolar bone, and the bone 

 marrow of the jaw are injured, and the development of the entire jaw 



retarded. 



In humans even mature teeth may undergo atrophy and fall out on 

 exposure to strong doses of roentgen rays or radium. Injury to the teeth 

 appears to be due to the absence of saliva. Exposure of the pregnant 

 uterus to radiation has been known to inhibit dental development in the 

 child, but this is probably the result of interference with general growth 

 (74). 



HEMATOPOIETIC SYSTEM 



A discussion of the influences of irradiation upon cellular elements of 

 the blood must be divided up into a study of the various cells and the 

 organs concerned with the production of these various elements (369). 



The spleen and lymph nodes have long been known to contain large 

 numbers of lymphocytes sensitive to radiation (147, 165, 170, 236, 334). 

 Akaiwa et al. (6) studied the changes in excised popliteal lymph nodes of 

 the rabbit at various intervals following one-third to 2 human skin 

 erythema doses (0.08 A). The destructive changes brought about were 

 proportional to the dose. Within the first 12 hr. there is an increase in 

 size of the lymph node with nuclear disintegration, especially of the 

 lymphocytes, and decomposition of germinal centers with "cellular 

 infiltration, congestion, and exudation." Mitotic figures were seen only 

 following small doses but not after large doses. In the period between 

 12 hr. to 5 days after irradiation, there is phagocytosis and removal of 

 debris. After 5 days the damaged areas are filled in with connective- 

 tissue cells. This article and its illustrations are worth a careful perusal. 

 The changes in the spleen resemble in every detail those noted above for 

 the lymph node (65, 147, 208, 220, 369, 371, 372). The spleen may be 

 reduced to a stroma remnant showing nothing but connective tissue, 

 reticulum, and blood vessels (84, 334) after intensive irradiation of either 

 single massive or repeated dosage. 



The extreme sensitivity of the lymphocytes has been shown by Murphy 

 (249, 250) who was able with repeated very small doses of low-voltage 

 roentgen radiation, to deplete the lymphocytes in laboratory animals 

 without changing appreciably the numbers of other circulating cells in 

 the blood. The lymphocytes were depleted to their lowest level about 

 18 hr. after the last irradiation. The return to normal depended upon 

 the amount of dosage in some cases requiring 1 to 2 months. The dosage 

 used caused no appreciable change in the bone-marrow cells of these mice. 



