254 BIOLOGY. [Book ii. 



analogous to Mbernation. In the mammifers it is a very limited 

 region, situated at the origin of the spinal marrew, in the 

 rachidian bulb on a level with the V of grey substance, compre- 

 hended in the posterior angle of the fourth ventricle. Now the 

 integrity of this region is so necessary to the accomplishment of 

 the respiratory movements that a section performed at this point 

 at once destroys both respiration and life. This celebrated point 

 in the spinal marrow is known as the nodus vitalis. Galen had 

 already pointed it out ; but only in our days, through the miuute 

 researches of Flourens, has its precise position been determined. 



In reptiles, the cutting of the nodus vitalis has a much less 

 striking effect than in the mammifers ; it effectually destroys 

 the respiratory movements, but does not immediately kUl the; 

 animal, a result very easy to comprehend, and which is attri- 

 butable to the feebler centralization, the smaller energy of the- 

 respiratory apparatus. Here the skin comes to the aid of the ' 

 lungs in a great measure ; whence batrachians are of all reptiles '< 

 those which survive the longest the cutting of the nodus vitalis. 

 This section besides, not only destroys the respiratory move- 

 ments, but also the whole of the voluntary movements of the 

 trunk, the members, and the head, since it interrupts most of 

 the conscious and voluntary communications between the brain 

 and nearly the whole of the other organs. 



Our object being, not to write a special treatise on physiology, 

 but rather to give a general idea of the process by which nutrition 

 is accomplished in the animal kingdom, we shall here terminate 

 this brief but adequate description of respiration. We have now 

 seen how the animal organism elaborates and absorbs the 

 materials which it borrows from the outer world ; how these 

 materials, having become assimilable, circulate throughout the 

 whole of the animal mechanism, and how atmospheric oxygen, 

 indispensable to the accomplishment of the primary phenomena 

 of nutrition, is absorbed. It remains for us to examine how the 

 organisms free themselves from the non-gaseous products of dis- 

 assimilation ; how also they fabricate, at the expense of the 



