EFFECTS OF LOWERING THE TEMPERATURE OF HOMOIOTHERMIC ANIMALS

¹ The Laboratory of Physiology, McGill University, Montreal

1. In the process of lowering the temperature of homoiothermic animals there is a progressive paralysis of the central nervous system as indicated by the gradual disappearance of voluntary and reflex actions. In recovery a definite order of reappearance of reactions occurs at different deep rectal temperatures as follows :—

Knee jerk and micturition reflexes ............... 16°

Achilles-thrust, and anal sphincter respiratory reflex ................ 17

Stepping reflex .......... 18

Shivering reflex .......... 19.5

Pinnae, vibrissae, flexion (of limb) and phonation reflexes ........ 20

Clawing and winking reflexes ................. 21.5

Head-shake, corneal, sneezing and swallowing reflexes .................................. 22

Light-reflex ........... ................. 23

Equilibrium, sight, smell and hearing ............ 27.5

Co-ordinated action (walking, etc.) ..... ....... 28

2. A special vibrissae reflex is described. This occurs readily at 20°.

3. In the initiation and subsidence of stepping movements at low temperatures, a definite march or order is observed.

4. A regular alternation between shivering and inspiration occurs between 22° and 24°.

5. Exaggerated tonic spasms of the postural musculature occurs when the temperature of the recovering animal stands between 21° and 2°.

6. In its progressive effects gradual cooling shows a strong similarity to the induction of anaesthesia, consciousness becoming abolished gradually and disappearing between 27° and 25°, while the ordinary reflexes are still maintained (24° to 19°). At low temperatures (19° or 20°) the pupil becomes constricted, to dilate widely just before death.

7. No condition resembling "artificial hibernation" can be said to occur when a homoiothermic animal such as the cat is reduced to very low body temperatures; a definite thermogenic ability is retained even at 19°, from which temperature spontaneous recovery is effected in 10 to 12 hours.

8. The activities of the heart, respiratory muscles and abdominal viscera are probably wholly responsible for an initial small rise in body temperature, but spontaneous shivering and stepping movements and an exaggerated muscular tonicity occurring above 20° are the chief factors in heat production and hence in recovery of the animal.

9. So far as can be determined, death from exposure to cold is due to paralysis of the respiratory centre in the medulla oblongata.