CHAP, xin RESPIRATORY RHYTHM 441 



A central organ, or, better, a complex of nerve centres, 

 interassociated, and constituting a small system ; 



(c) Afferent nerves capable of modifying, directly or indirectly, 

 the activity of the said centres. 



The motor nerves to the muscles, which normally determine 

 inspiratory expansion and expiratory retraction of the thorax, all 

 arise in the anterior roots of the spinal nerves. The motor nerves 

 to the scaleni emerge from the cervical tract, more exactly from 

 the second to the seventh nerves, and thus form the cervical and 

 brachial plexus, from which the branches to the muscles are given 

 off. The phrenic nerves which serve the diaphragm take origin 

 specially in the fourth cervicals, but are reinforced by fibres from 

 the third and also from the fifth pairs. The levatores costarum 

 longi et breves, the external and internal intercostals, and the 

 abdominal muscles receive nerves from the thoracic pairs of the 

 spinal cord : and, in particular, the ranii posteriores of the dorsal 

 nerves serve the levatores costarum : the intercostal nerves, the 

 muscles of the same name ; and the internal or anterior branches 

 of the intercostals, the muscles of the abdomen, which also receive 

 fibres derived from the first lumbar pair. 



Physiological proof of these morphological data is afforded by 

 the following experiments. A transverse section through the 

 spinal cord below the exit of the last intercostal nerve leaves all 

 respiratory movements entirely unaffected, while a cross-section 

 in the thoracic cord paralyses all the respiratory muscles, the 

 nerves of which arise below the section. When the lower part of 

 the cervical cord is transversely divided, i.e. above the first inter- 

 costal and below the exit of the fifth cervical nerves, all the 

 motor muscles of the ribs are paralysed (with the partial excep- 

 tion of the first two, which are raised by the scaleni) so that the 

 respiratory movements are effected almost exclusively by the 

 rhythmical activity of the diaphragm (Fig. 86, p. 229). If the 

 results of this operation are compared with those following the 

 section of the phrenic nerves (Fig. 202), they show the extreme 

 functional importance of the intercostal as compared with the 

 diaphragmatic muscles paralysis of the latter being in no way 

 dangerous to the life of the animal, save in the case of young 

 rabbits, in which the thorax is not sufficiently rigid, nor the 

 thoracic muscles sufficiently developed, to allow of ready compen- 

 sation for the failure of the diaphragm. These observations are 

 confirmed by those made on the human subject, which show that 

 after paralysis of the diaphragm the respirations become somewhat 

 more frequent, but are accomplished solely by the muscles to 

 the ribs with no active co-operation of accessory muscles. 



AVhen the section through the cord is made above the exit 

 of the third cervical nerves, as far as the apex of the calamus 

 scriptorius, all the respiratory muscles properly so-called are 



