THE RABBIT 453 



have no nuclei. (2) The temperature rjf the blood, instead 

 of rising and falling with that of the surrounding air or 

 water, is almost constant at about 38° C. This is expressed 

 by saying that the rabbit is a warm-blooded animal. The 

 heat is produced, not in the blood, but in the solid tissues, 

 particularly in the glands and muscles, its appearance 

 accompanying the activity of the tissue. The circulation 

 of the blood, however, keeps the temperature of different 

 parts of the body nearly the same. The regulation of the 

 temperature of the body as a whole is brought about by 

 alteration in the production of heat and in the rate at 

 which it is lost. The principal means of increasing the 

 production of heat is the activity of the muscles. Shivering 

 is an example of this. Loss of heat is promoted by in- 

 creased circulation in the skin and by sweating, which 

 absorbs heat in the evaporation of the sweat. 



The brain of the rabbit resembles that of the frog in the 

 main outlines of its structure, but there are 

 system! considerable differences in detail between the 



two. The most conspicuous part is the cere- 

 brum, which consists of two very large cerebral hemi- 

 spheres divided by a deep cleft or median fissure, at the 

 bottom of which they are joined by a bridge known as 

 the corpus callosum, composed of nerve fibres, nearly all 

 of which run transversely. The surface of the hemispheres 

 is almost smooth, but there can be seen on it faint indica- 

 tions of some of the furrows or sulci which in man are 

 deep and numerous and divide the surface into convolu- 

 tions. At about the middle of its length each hemisphere 

 is marked at the side by a shallow groove known as the 

 lateral or Sylvian fissure, which separates a temporal lobe 

 from the rest. On the under side a longitudinal rhinal 

 fissure marks off the frontal and temporal lobes from a 

 region median to them known as the rhinencephalon, which 

 consists of a hippocampal lobe behind and the olfactory 

 lobe in front. The latter consists of the olfactory tract 

 and the olfactory bulb which projects in front beyond 

 the frontal lobe. The thalamencephalon is overhung 

 and hidden by the cerebral hemispheres. Its thick' 

 sides form two large optic thalami, and from the hinder 

 part of its thin roof the pineal stalk passes backwards to' 



