816 SPECIAL PHYSIOLOGY. 



of the face belonging to the fifth cranial pair, and the cutaneous nerves 

 of the body generally. The nasal nerves and the laryngeal branches 

 of the pneumogastric, excite expiratory movements, as in sneezing 

 and coughing. The nervous centre which governs the respiratory 

 movements, is a limited portion of the gray matter of the medulla 

 oblongata opposite the roots of the pneumogastric nerves, as has been 

 proved experimentally (p. 281). The so-called vital knot, at the 

 back of the medulla, corresponds with the interval between the oc- 

 cipital bone and the arch of the axis ; in this space an animal may be 

 suddenly killed by introducing a sharp, strong knife, so as to pith it, 

 or divide the medulla. The efferent or motor respiratory nerves are : 

 the phrenic nerves, which supply the diaphragm ; the intercostal 

 nerves, which supply, amongst others, the muscles of that name ; the 

 lung thoracic, or so-called external respiratory nerves of Bell, which 

 are distributed to the serrati muscles ; the spinal accessory nerves, 

 which supply the trapezii muscles; and the facial nerves. In extreme 

 respiratory efforts, other motor nerves are of course concerned. The 

 reflected stimulus is conveyed to the roots of these nerves, along a 

 particular tract of the spinal cord, situated behind the anterior roots 

 of the spinal nerves, and descending along the lateral columns of the 

 cord, from between the olivary and restiform bodies ; this is the re- 

 spiratory tract of Sir C. Bell, but, except as a path of special conduc- 

 tion, it has no respiratory influence. It is remarkable that the phrenic 

 nerves, which supply the diaphragm, the most active muscle in inspi- 

 ration, arise from the cervical plexus, and therefore form a part of the 

 cord much higher than the roots of the highest intercostal nerves. 

 Hence it happens that in certain injuries or diseases of the spinal cord, 

 when the seat of these is above the origins of the intercostal, but below 

 those of the phrenic nerves, costal respiration may be entirely abol- 

 ished, whilst diaphragmatic breathing goes on. In such cases, how- 

 ever, death ultimately occurs from asphyxia, caused by the slow exuda- 

 tion of fluid into the lungs. 



The respiratory movements being regarded as reflex or excito-motor, 

 the 'first act of inspiration performed at birth, is said to be induced by 

 the stimulus of cold acting on the excitable extremities of the fifth 

 cranial nerves, which supply the nasal fossae and the skin of the face, 

 and also on those of the cutaneous nerves of the whole body. Cold, 

 or a smart blow applied to the surface of the body of an apparently 

 still-born infant, will sometimes excite inspiration; and, moreover, if 

 the face of an infant be protected by warm covering, the first inspira- 

 tory act may be postponed. (Marshall Hall.) Once established, the 

 reflex respiratory movements are believed to be excited by a peculiar 

 stimulus, accompanying the sum of disagreeable sensations included 

 in the feeling of want of breath. By some, it is supposed that the 

 venous blood, deprived of oxygen, and loaded with carbonic acid gas 

 and other effete matters, owing to the periodic interruption to the pro- 

 cess of oxygenation, may be the cause of some of those disagreeable 

 feelings, and may periodically stimulate the medulla and spinal cord, 

 and so rhythmically excite the motor nerves of inspiration ; according 





