ted, there were profound effects. But in some cases the acute phase was success- 

 fully negotiated and complete recovery seemed in sight, when a crisis of depression 

 occurred, of obscure origin and usually fatal. (This sequence of events is not un- 

 familiar in human cases of hyperthermia). 



A very successful type of rabbit was a black female — No. 60 — exposed en- 

 tirely in the shade. Rectal temperatures were:- initial, 104. 1°F rising to 112. 7°F 

 and then, with cooling, falling to 101.0°F. Haemoglobin had an initial value of 105% 

 gradually falling to 92% at which level it remained for 28 minutes, then rose to 105% 

 at the rectal temperature peak and to 113% at the conclusion of cooling treatment. 

 Blood pressure was not estimated. Carbon dioxide and air volumes exhaled were ac- 

 cording to expectations. The heart beats of this rabbit were counted with a binaural 

 stethoscope (by tapping on a sheet of paper for ten seconds) on twelve occasions 

 during the experiment. Initially the heart beats were 288 per minute and loud, in- 

 creasing gradually with increase of body temperature to 350 beats per minute, loud, 

 continuing loud and rapid until just after the rectal temperature peak, when the beats 

 were 350 per minute and quiet. After cooling the beats were 300 per minute and 

 loud again. This rabbit recovered without complications. 



Rabbit No 73 had a similar shade treatment, but is chosen because of the record 

 of blood pressure. Initially rectal temperature was 104. 2°F and blood pressure was 

 64.0 mm. Hg. rising to 80.0 mm. Hg. at rectal temperature 109.0°Fand falling to 42.0 

 mm.Hg. at the rectal peak of temperature (111.0°F). In the early stages of cooling 

 the blood pressure rose abruptly to 120.0 mm.Hg. and then fell, equally suddenly, to 

 20.0 mm.Hg. with recovery to 80.0 mm.Hg. about ten minutes after the cessation of 

 the cooling treatment. This rabbit also recovered completely, without complications. 



Several rabbits developed cyanosis, apnoea, and almost inaudible heart beats 

 at the apex of rectal temperature, i.e. between 111.0°F and 114.0°F. In one case, 

 the rabbit became cyanosed, stopped breathing, and was treated by ice pack, with- 

 out avail. In this animal some three minutes after apparent death (cessation of res- 

 piration with unconsciousness) the heart was still beating at 80 beats per minute. 



One rabbit of this group showed a blood pressure fall from 70.0 mm.Hg. initially, 

 to 40.0 mm.Hg. at rectal temperature 105. 6°F then, at rectal temperature 107. 8°F re- 

 covered to 70.0 mm.Hg. and at rectal temperature 109. 8°F was still 60.0 mm.Hg. Then 

 the blood pressure fell suddenly to 36.0 mm.Hg., made a jerky recovery to 65.0 mm.Hg. 

 at the rectal temperature peak (112. 0°F) and fell to 10.0 mm.Hg. in less than five 

 minutes. Ice pack treatment was unavailing, death occurred in a few minutes. With 

 sudden death at the peak of rectal temperature all the body systems, respiratory, 

 circulatory and nervous, seemed to be simultaneously depressed, probably from the 

 effects of heat on the nerve cells of the brain, including the respiratory and other 

 centres, and on the regulatory centre in the hypothalamus. This problem of brain 

 lesions will be approached later. 



For the problem of delayed death some details of rabbit No. 22 will be con- 

 sidered. This female albino rabbit was restless and struggled throughout the ex- 

 posure, entirely in the shade. Initially haemoglobin was 95.0% at rectal temperature 

 104. 2°F when the blood pressure was 75.0 mm.Hg. The haemoglobin rose to 114% at 



190 



