JSI? Analogy between Vegetables and Aimnals 



to variations of temperature. The function of respiration 

 is partly under the control of the will, and it is partly inde- 

 pendent of it. For instance, we can breathe slowly or 

 rapidly, and, to a certain degree, we can stop the breathing. 

 The principal agent concerned in carrying on the respiratory 

 function is the diaphragm, which is a large muscle separating 

 the chest from the cavity of the abdomen. I have already 

 mentioned that no muscle will contract, unless its fibres be 

 excited by the application of some stimulus. Tlie involuntary 

 muscles contract in consequence of the stimulus being made 

 to act directly upon them. The heart, for example, is stimu- 

 lated by the contact of the blood, the stomach by that of the 

 food, and the bowels by the chyle and faeces. The stimulus 

 by which the action of the voluntary muscles is produced 

 is volition, which is sent to them from the brain through the 

 medium of the nerves. Now, it is clear that the diaphragm 

 does not, like the heart and stomach, contract, owing to any 

 substance being immediately applied to it Neither do its 

 ordinary contractions proceed on the principle of voluntary 

 muscles ; for volition is exercised upon it only very occasion- 

 ally. It must, therefore, have some stimulating power trans- 

 mitted to it from some source which does not supply the 

 other muscles. Now, it is well known that the diaphragm, 

 besides having nerves of sensation and volition whence it 

 derives its voluntary power, receives likewise a particular 

 set of nerves from the middle of the spinal cord. These 

 nerves are independent of the brain. They are incapable 

 of communicating perception or volition, and have only the 

 power of exciting the fibres of the diaphragm to contract. 

 It is by the peculiar agency of these nerves that the con- 

 tractions of the diaphragm can go on without our con- 

 sciousness. Hence it is that we are able to breathe during 

 ^leep, and during insensibility from disease. On the banks 

 of the Ganges there grows a plant called the J/edysarum 

 gyrans [Desmodium gyrans Decandolle'], The leaves are 

 continually in motion. These motions are connected with 

 the function of respiration ; and we are informed by Sir 

 J. E. Smith that they will continue when the plant is 

 removed from the light and every external agent. From 

 this circumstance, they must depend upon some internal 

 cause ; and to what cause can we refer them, except nervous 

 power, or something very closely similating it ? There seems, 

 I thiiuk, very little difference between the motions of the 

 leaves of the Desmodium gyrans and the ordinary contrac- 

 tions .of the diaphragm. They both originate from some 

 internal stimulus : they are both concerned in the function 



