RESPIRATORY MOVEMENTS. 141 



the Spinal Cord the impression of imperfectly arterialized blood circulating in 

 these, such as the Pneumogastric is believed to transmit from the lungs. It 

 will hereafter be shown, that an impression of a corresponding kind is more pro- 

 bably the cause of the sense of hanger and thirst than any which originates 

 in the stomach alone ( 437). The Motor or Efferent nerves concerned in the 

 function of Respiration, are those which Sir C. Bell has grouped together in 

 his respiratory system. The most important of these, the Phrenic, arises from 

 the upper part of the Spinal Cord ; the Intercostals much lower down ; whilst 

 the Facial nerve and the Spinal Accessory, to the latter of which, as will here- 

 after be stated ( 229), the motor powers of the par vagum are chiefly due, 

 take their origin in the Medulla Oblongata itself. But we must not decide 

 upon the connection of a particular nerve with a particular segment of the 

 Spinal Cord, simply because it diverges from it at that point. It has been 

 shown that, in the Mollusca, a nerve passing to, or proceeding from, one gang- 

 lion, frequently passes through or over another which lies in its course ; and, 

 in the Articulata, this is a still more constant occurrence. It is by no means 

 improbable, then, that the connection of the intercostal nerves is really in part 

 with the gray matter of the Medulla Oblongata ; at any rate, such a connection 

 has not been -disproved. The white columns of the Spinal Cord consist of 

 fibres, 'which bring the spinal nerves into connection, not only with the brain, 

 but also with other segments of the ganglionic portion of the cord, being analo- 

 gous in function, not merely to the distinct fibrous tract of the ventral column 

 of the Articulata, but also to the fibrous bands that connect the ganglia them- 

 selves. As the Medulla Oblongata, in Vertebrate animals, is the chief centre 

 of the actions of Respiration, it can scarcely be doubted that all the nerves con- 

 cerned in that function have a direct structural connection with it. 



186. That the Respiratory movements, as ordinarily performed, are essen- 

 tially independent of the Will, appears not only from our own consciousness, 

 but also from cases of paralysis ; in some of which the power of the will over 

 the muscles has been'lost, whilst the movements have been kept up by the 

 reflex action of the medulla oblongata or respiratory ganglion ; whilst in 

 others, some of the respiratory muscles have been motionless during ordinary 

 breathing, and yet have remained under the power of the will. Such cases 

 are mentioned by Sir C. Bell, in. the appendix to his work on the Nervous 

 System. That consciousness is not a necessary link in the chain of causes 

 that produce the respiratory movements, we are enabled to judge from the 

 phenomena presented by the human being in sleep and coma, by anencepha- 

 lous foetuses, and by decapitated animals. Further, Dr. Ley* has put on re- 

 cord a case, which confirms this particular inference, just in the same manner 

 as the cases already related confirm the general doctrine of the non-existence 

 of sensibility in the Spinal Cord. He had under his care a patient in whom 

 the par vagum appeared to be diseased ; the lungs suffered in the usual way 

 in consequence, and the patient had evidently laborious breathing ;' but he 

 distinctly said that he felt no uneasiness in his chest. The experience of 

 every one informs him, that Respiratory movements are partly under the con- 

 trol and direction of the will, though frequently unrestrainable by it. In 

 ordinary circumstances, when the blood is being perfectly aerated, and there 

 is a sufficient amount of arterial blood in the system to carry on the functions 

 of life for a short time, we can suspend the respiratory actions during a few 

 seconds without any inconvenience. If, however, we endeavour to prolong 

 the suspension, the stimulus conveyed by the excitor nerves to the Medulla 

 Oblongata becomes too strong, and we cannot avoid making inspiratory efforts ; 

 and if the suspension be still further prolonged, the whole body becomes agi- 



* On Laryngismus Stridulus, p. 417. 



