THE 



AMTOMY OF YERTEBRATES. 



CHAPTER XIII. 



CHARACTERS OF H^MATOTIIERMA. 



§ 121. Thermogejious Conditions. — Life is attended \\\\\i con- 

 stant molecular change ; and such vital motion becomes converted 

 directly, or through intermediate modes of chemical or electrical 

 force, into that of heat ; the chief chemical action preceding or 

 producing the calorific force being due to the introduction of oxy- 

 gen by air or food into the body, where it operates in a manner 

 analogous to combustion. 



The evolution of animal heat more directly relates to the 

 amount of air inspired in a given period, and to the rapidity with 

 which the oxygenated blood is conveyed to the tissues. 



In these the molecular changes are governed by the nervous 

 system, and whatever tends to paralyse the nervous force operates 

 in the same degree in arresting those molecular movements on 

 which more directly depends the evolution of heat. In this act 

 the nervous system is accordingly concerned, in so far as it 

 influences the exercise of the muscular movements. 



In the Hoimatotherma, or Warm-blooded Vertebrates, the at- 

 mosphere is directly inspired and applied to a vascular surface 

 Avhich, in proportion to the bulk of the body, is much more exten- 

 sive than in any of the HcEmatocrija. For mechanical convenience 

 the respiratory surface is closely packed, in small compass, by 

 subdivision of the pulmonary ca^dty into countless minute cells, 

 giving to the lung a spongy texture, obliterating all trace of a 

 visible or conspicuous cavity. 



The whole of the venous blood is propelled over this extensive 

 but compactly disposed capillary area by successive contractions 

 of a special ventricle, receiving it from a distinct auricle, and the 



VOL. II. B 



