THE NERVES OF THE RESPIRATORY MUSCLES. 465 



The same dual supply should prevail, however, in the ex- 

 ternal respiratory muscles. "When an intercostal nerve is cut," 

 continues Professor Foster, "no active respiratory movements 

 are seen in the intercostal muscles of the corresponding space, 

 and, when the spinal cord is divided below the origin of the 

 seventh cervical spinal nerve, that is, below the exits of the 

 roots of the phrenic nerves, costal respiration ceases, though 

 the diaphragm continues to act, and that with increased vigor/ 7 

 Again is the co-ordinating factor absent. The twelve inter- 

 costal nerves are now considered as mere motor nerves and 

 lack afferent fibers capable of accounting for the impulses that 

 call forth counter-co-ordination impulses from the bulb. Each 

 nerve, however, is thought to be accompanied by a filament 

 from the sympathetic also an efferent nerve and occasion- 

 ally (only on the left side, according to Swan) with commu- 

 nicating branches from the hypoglossal. We have not met, as 

 yet, muscles supplied with two motor nerves excepting when 

 the associate nerve was vagal. On the other hand, so important 

 a function as adjustment of the intercostal muscles to corre- 

 sponding functions would hardly be satisfied by a set of "occa- 

 sionally present" filaments. We are, therefore, inclined to 

 consider what is now thought to be "sympathetic" filaments 

 as themselves hypoglossal filaments, to which the adventitious 

 ones would stand as anomalous duplications. 



Indeed, close examination reveals the following facts: 

 each intercostal artery receives a vasomotor filament from the 

 ganglion immediately above it, while each intercostal nerve 

 receives two thicker branches from the ganglion below it. The 

 manner in which the decussations occur and the way in which 

 their distribution is disposed suggest that two nerves, as is the 

 case higher up, are insheathed in each ganglion. Under these 

 circumstances, hypoglossal fibers from the medulla would insure 

 the co-ordinating attributes to the muscles, in conjunction with 

 the latter 7 s recognized general motor nerves, the intercostals 

 thus normally accounting for the precision with which the 

 latter fulfill their functions, which otherwise remains unex- 

 plained. 



That considerable confusion now exists regarding the part 

 taken by ganglionic branches of communication is suggested 



