Thirdly, If we suppose the diaphragm to have finished its contraction, and consequently the thorax to be raised, and the abdomen to be distended, expiration must succeed, for three reasons; viz. because the contraction of the diaphragm has checked the vibrations in its fibres, and therefore has disposed them to relaxation; because the elasticity of the cartilages of the ribs tends to restore the thorax to a more depressed state; and because the fibres of the abdominal muscles are excited to vibrations and contraction by their distention. Suppose now expiration to be completed, and the vibrations, which descend into the diaphragm, from the second and third source of motory vibrations, will renew its contraction, and consequently the action of inspiration; and so on alternately.
Fourthly, The diaphragm is the more disposed to receive vivid vibrations in its fibres, from its nearness to the heart, and the warmth in which it is always kept. It deserves notice here, that the diaphragm receives its nerves from the cervical pairs, not from the eighth pair and intercostal, as the neighbouring parts, the heart, stomach, &c. do. Perhaps it was requisite, that it should derive its nerves from the spinal marrow, since the other muscles of respiration do. The nerves of the diaphragm are elongated during its contraction, the contrary to which happens in the nerves of other muscles. They are also kept in constant motion by the constant motion of the thorax, through which they pass; and this perhaps may contribute to the perpetual activity of the diaphragm.
Fifthly, The intercostal muscles are disposed to alternate contractions for the same reasons as the diaphragm, though these reasons do not hold in the same degree; however, since the contraction of the diaphragm, and consequent distention of the abdomen, must raise the thorax, the contraction of the intercostal muscles must from the first be synchronous to that of the diaphragm; and association will soon generate a permanent disposition to this synchronism. It follows according to this theory from the vicinity of the internal intercostal muscles to the external ones, that they ought to have the same action. And the same conclusion appears to result from anatomical considerations.
Sixthly, It may however be supposed, that the diaphragm and intercostal muscles are, as it were, fatigued by constant action; and that they incline to sympathize in inactivity with the rest of the muscles, especially during sleep. But then the accumulation of blood in the lungs, which the intermission of respiration must occasion, after it has once had a free passage through the lungs by means of respiration, will produce so much uneasiness, i.e. such vigorous vibrations, in the lungs, and particularly in their external membrane, which is continuous to the pleura, as will renew respiration.
It is agreeable to this, that if very young children sleep upon
