RESPIRATION. 
375 
hyo-epiglottideus, thyro-arytenoideus, and crico-arytenoideus la- 
teralis ; — the first muscle as the principal agent in elevating the 
epiglottis, and the other two enlarging the glottis by separating 
the arytenoid cartilages. An increased degree of irritability in 
the aryteno-epiglottideus and thyro-epiglottideus muscles, pro- 
ducing an irregular contraction of them, by which the epiglottis 
may be partially depressed at a period of the respiratory process 
when its full elevation is essentially required, would produce a 
similar obstruction to respiration. 
The vessels and nerves distributed to muscles must be sound 
for the due performance of their contraction. Any obstruction or 
defect in the circulation either of the blood or nervous fluid sup- 
plied to them is attended with a proportionate change of func- 
tion. A mediate state of total paralysis or of complete spasm, 
existing in the laryngeal muscles arising from an alteration in the 
nerves or bloodvessels, would change the laryngeal murmur to 
whistling or roaring . 
The late Mr. John Field experimented on the muscles of the 
larynx by dividing the recurrent nerve of the par-vagum on one 
side, and found the animal, immediately after the operation, to be 
a roarer in a very extensive degree, and his breathing very diffi- 
cult. His respiration afterwards got better ; but at the end of 
four years he was still a bad roarer, and, on dissection, presented 
a wasting of the laryngeal muscles on the side operated upon. 
The loss of substance in the muscular structure in this instance 
arose from the deprivation of the nervous influence, and not, as is 
by some conjectured, in consequence of an inactive state of the 
muscles. The cause of roaring was plainly a palsy of the dilators, 
and consequent collapse of the larynx on that side. The effect 
upon the respiration was much the same as arises from the thick- 
ening of the lining membrane of the larynx — a contraction and 
diminution of the passage, which is the true and real cause of 
roaring in all its modifications. 
The laryngeal nerves supply the constrictor muscles of the 
larynx with motor power; the recurrent branches those of the 
dilators : all proceeding from the par-vagum, yet supplying an- 
tagonist muscles with power of motion ; and so beautifully ad- 
justed is the balance between them in their normal action, as to 
admit of no inconvenience or hindrance to the free and easy ad- 
mission and expulsion of air to and from the lungs. But should 
the nervous influence be increased on the one hand, or diminish- 
ed on the other, the mutual relationship in operation between 
them is impaired ; and an alteration in the laryngeal murmur is 
the consequence. This condition of the muscles may exist with 
or without apparent change of structure in them ; but undoubt- 
