xiii EESPIEATOEY EHYTHM 443 



are innervated by the vagus, partly to the striated fibres derived 

 from the muscles of the diaphragm, which penetrate the 

 parenchyma of the lungs, to invest the large alveoli, and are 

 innervated from the spinal nerves. When the vagus is stimulated 

 in the neck of this animal, a curve of slow prolonged contraction 

 is obtained from the lung, exactly similar to that served up by 

 smooth muscle. When, on the other hand, the spinal cord is 

 excited, a rapid pulmonary contraction results, which is evidently 

 due to the striated muscles. 



The physiological function of the smooth muscles of the- 

 ^bronchi, and of the constrictor and dilator nerves which serve 

 them, is not yet fully explained. It seems obvious that they give 

 greater resistance to the bronchial walls, and reinforce this resist- 

 ance by their contraction, when the negative intrathoracic pressure 

 falls too low, during forced inspiration. Probably the development 

 of the pulmonary emphysema is promoted by atony, or by the 

 paresis or paralysis of the smooth muscles. 



II. As a whole the central mechanisms, from which the 

 several motor nerves to the respiratory muscles receive their 

 rhythmical impulses, must be excessively complicated, seeing that 

 the co-ordination of the inspiratory and expiratory movements, 

 i.e. the harmonious and synergic contraction of the muscles which 

 alternately expand and contract the thorax, depend upon them. 

 The immediate centres for the motor respiratory nerves must, 

 however, be distinguished from the true controlling and co- 

 ordinating respiratory centre. The former lie in the cervico- 

 dorsal tract of the spinal cord, and are formed from the grey 

 matter of the anterior horns, which contains the nerve cells of 

 which the nerves to the respiratory muscles are the prolongation ; 

 the second is situated in the medulla oblongata, and has probably 

 no direct influence upon the muscles, but is confined to exciting 

 and regulating the functions of the former. 



When the brain is extirpated to the level of a plane which 

 . passes along the inferior limit of the pons, or when a section is 

 made at the level of this plane, it will be seen that after temporary 

 disturbance the animal continues to breathe spontaneously, in a 

 regular and perfectly co-ordinated manner. This experiment 

 proves that the co-ordinating centre for the respiratory movements 

 does not lie higher than the spinal bulb. When, on the contrary, 

 the bulb is divided from the cervical cord at the apex of the 

 calamus scriptorius by a transverse section, the respiratory move- 

 ments ipso facto come to a standstill. This proves that the 

 respiratory centre lies within that section of the bulb which is 

 situated between the two planes of division indicated. 



Which portion of the bulb is it, however, which represents 

 the respiratory centre ? The experiments directed towards the 

 localisation of this centre have a very involved history, which 



