236 INTRODUCTION TO NEUROLOGY 



simply absorbed from the surrounding medium by the exposed 

 surfaces. In all but the lowest animals there is a blood-vascular 

 system by means of which the oxygen absorbed at the surface is 

 transferred to the deeper tissues. In insects, however, this re- 

 sult is obtained chiefly by a different apparatus, namely, a sys- 

 tem of air tubes (tracheae) which ramify among the tissues and 

 supply oxygen directly to the functioning cells. In most water- 

 breathing animals a portion of the surface of the body is lamel- 

 lated and vascularized to form gills to facilitate the absorption of 

 oxygen by the blood-stream, and in air-breathing vertebrates 

 lungs are developed to accomplish the same result. The nervous 

 mechanisms of respiration will differ in all of the cases cited 

 above, and it is only in mammals that we shall here consider the 

 details of this mechanism. 



In ordinary breathing, inspiration is effected by actively in- 

 creasing the volume of the thoracic cavity and thus creating a 

 suction through the trachea, while expiration is the result of the 

 passive return of the organs involved to their former positions 

 by reason of their own elasticity. The muscles involved in 

 inspiration belong to two groups: (1) the internal apparatus, i. e., 

 the diaphragm, and (2) the external apparatus, the intercostal 

 and other muscles of the body wall. These are all somatic 

 muscles. In forced respiration various other muscles act in an 

 accessory way during both inspiration and expiration. 



The diaphragm is innervated by the phrenic nerve, which 

 takes its origin from the fourth and fifth cervical spinal nerves; 

 and the intercostal muscles are innervated by ventral spinal 

 roots arising successively from all thoracic segments of the spinal 

 cord (Fig. 112). The accessory muscles are in part somatic 

 muscles of the abdomen and shoulder and in part special vis- 

 ceral muscles of the head, particularly those of the glottis 

 (innervated by the vagus) and of the nostrils (innervated by the 

 VII cranial nerve). 



The anatomical relations just described imply that, although 

 respiration is a visceral function, in mammals the necessary 

 movements for ordinary breathing are performed by somatic 

 muscles. This is not true in fishes. Here the organs of respi- 

 ration (gills) are strictly visceral structures innervated by vis- 

 ceral components of the cranial nerves, whose cerebral center is 



