28o DESIGN IN NATURE 



Notwithstanding the large amount of attention which has been bestowed upon them from early times, the accounts 

 given of them are by no means satisfactory. This arises from three causes : — 



(a) The prevailing views as to muscular action. 



(b) The high rule ascribed to elasticity — it being elevated to the position of a prime motor. 



(c) The number of muscles employed. 



The rhythmic movements of the nostrils are due to the compressors and dilators of the nose. They are as a 

 rule feeble, but there are cases in which they are well marked, and they can always be witnessed in forced respira- 

 tion. They are seen to advantage in the horse and in the sea-mammals, such as the walrus, seal, sea-bear, manatee, 

 porpoise, whale, &c. During each inspiratory effort, Professor Michael Foster observes, " The nostrils are expanded, 

 probably by the action of the dila tores naris, and thus the entrance of air faciHtated. Their return to their previous 

 condition during expiration is effected by the elasticity of the nasal cartilages, assisted perhaps by the compressores naris." 



The rhythms of the glottis are due to a complicated system of muscles found on the larynx. Like those of the 

 nostrils, they are faint in ordinary breathing but always perceptible and pronounced in forced breathing. They 

 are more or less mixed up with the action of the vocal chords and the production of voice, and on this account require 

 a separate description. 



The rhythms of the lungs are not quite made out, but the presence of large quantities of pale unstriated 

 muscular fibres on the smaller and larger bronchi aflord presumptive testimony of their existence. In the interior 

 of the bronchi, ciliary movements which are essentially rhythmic in their nature can readily be made out. 



The rhythmic movements occurring in the diaphragm and in the muscles forming the walls of the chest and 

 abdomen practically run into each other, and will be best considered together. 



As all the air which passes into and out of the lungs must first find its way through the glottis, its anatomy is 

 no less important than its physiology (Plate Ixxvii., Figs. 7 and 8). 



In the upper part of the larynx and stretched across its opening are ligamentous bands known as the vocal 

 chords ; the two upper or false chords being covered with thick folds of mucous membrane, and the two lower or 

 true chords being covered with folds of very thin mucous membra.ne. The lower or true alone are concerned in the 

 formation of voice. 



Both sets of chords (false and true) are attached to a fixed point between the arytenoid cartilages and posteriorly 

 to the movable arytenoid cartilages. Small muscles, which arise from the thyroid and cricoid and are attached to 

 the arytenoid cartilages, have the power of separating and bringing together the points to which the vocal chords 

 are attached posteriorly so as to open and close the chink or sht between them called the rima glottidis. It is through 

 the rima glottidis that air is admitted into the lungs, and through it all the air expelled from the lungs must pass. 



" If," says Professor Austin Flint, " the glottis be exposed in a living animal, certain regular movements are 

 presented, which are synchronous with the acts of respiration. The larynx is opened at each inspiration by the 

 action of the muscles referred to above, so that the air has a free entrance to the trachea. At the termination of 

 the inspiratory act these muscles are relaxed, the vocal chords fall together by their own elasticity, and in expira- 

 tion the chink of the glottis returns to the condition of a narrow sht. These respiratory movements of the glottis 

 are constant, and are essential to the production of air in proper quantity into the lungs. The expulsion of air from 

 the lungs is rather a passive process, and tends in itself to separate the vocal chords, but inspiration, which is active 

 and more violent, were it not for the movements of the glottis would have a tendency to draw the vocal chords 

 together." 



Referring to the diaphragm he says : " From the great increase in the capacity of the chest produced by the 

 action of the diaphragm and its constant and universal action in respiration, it must be regarded as by far the most 

 important and efficient of the muscles of inspiration. Hiccough, sobbing, laugHng, and crying are due mainly to 

 the action of the diaphragm, particularly hiccough and sobbing, which are produced by spasmodic contraction 

 of this muscle, generally beyond the control of the will." ^ 



The diaphragm, which next to the heart is the most remarkable muscle of the body, is deserving of special con- 

 sideration. It is endowed with characteristic rhythmic movements, which enable it alternately to increase and 

 diminish the thoracic and abdominal cavities. It is a dome-shaped muscle which separates the chest from the 

 abdomen, and its muscular fibres run from its attachments— internal circumference of the lower border of the thorax 

 -to its central tendon in curved lines-the fibres tending to cross at right angles and at every degree of obliquity 

 as m the ventricles of the heart, the stomach, and bladder (Figs. 45 and 46) .^ 



I H^/ f'^l'^™'' "' Human Physiology," by Professor Austin Flint, Belleyue Medical College, New York, p. 116, &c. 

 1 he dmphragm is attached m front to the ensiform cartilage, on either side to the inner surface of the costal cartilages and bonv nortions of 

 the SIX or seven inferior ribs, and behind to the ligamentuni areuatuni externum and internum. ^ poitions ot 



Inasmuch as the serratus posticus inferior is inserted into the last three or four ribs and the quadratus lumborum into the last rib it i, thn„<,l,t 

 by some that these muscles act as muse es of inspiration, even in calm breathing, by affording fixed points to the diaphragm when it eontr.AT^ T 

 am disposed lu regard all the muscles which pull down the ribs as muscles of expiration, ■" * ' ^ " eontiaets. I 



