2/6 DESIGN IN NATURE 



PLATE LXXVII (continued) 



i, the ventricle with the false vocal chord above it ; k, interior of trachea conducting to the lungs. The narrow passage or chink 

 (rinia glottidis) through which the air passes during respiration and the formation of the voice is clearly shown (after Thomson). 



Fig. 8.— Three laryngoscopic views from life of upj)er aperture of larynx, showing vocal chords, glottis, and surrounding parts 

 in different states. 



A. Shows the glottis and vocal chords duriui,' tlie emission of a high note in singing. B. In natural inlialation of air. C. In 

 inhaling a very deep breath. Diagrams D, E, P, show horizontal sections of glottis, positions of vocal ligaments, and arytenoid 

 cartilages in states corresponding to A, B, C. The same letters apply to the same parts as in A, B, C (after Sappy). 



A, B. a, Base of tongue ; h, upper free part of epiglottis ; c, tubercle or cushion of ditto ; d, portion of anterior wall of pharynx 

 behind larynx ; e, swelling caused by cuneiform cartilage; /, ditto corniculum ; g, tip of arytenoid cartilages; h, inferior or true vocal 

 chords forming lips of rima glottidis or breathing aperture ; i, superior or false vocal chords with ventricle of larynx between. 



C. i, anterior wall of receding trachea ; k, beginning of the two bronchi beyond the bifurcation (after Czermak). 



In the fish distinct respiratory rhythmic movements are witnessed. The fish is constantly engaged in apparently 

 swallowing water — this water being made to flow in rhythmic waves over the gills, which consist of a framework 

 on which is arranged a congeries of deKcate capillary blood-vessels containing blood. A greater quantity of water, 

 and of the air which is in solution in the water, is thus made to pass over the gills in a given time. As a result 

 more oxygen passes from the air into the blood of the fish, and more carbonic acid out of it than would otherwise 

 be possible. 



The menobranchus, one of the water hzards, also displays rhythmic respiratory movements. This curious 

 creature is provided vsdth gills in the shape of six feathery-looking tufted structures, three on each side of the head. 

 These structures are composed of a central portion or midrib with an infinite number of fine capillaries containing 

 blood diverging from it (feather-fashion) on either side. The lizard causes the gills to wave gently backwards and 

 forwards in the water, with the result that a maximum of the oxygen contained in the air in solution in the water 

 is made to pass over the capillaries into the blood and a maximum of carbonic acid is made to pass out of the blood 

 in the capillaries into the water. The rhythmic movements of the gills, or branchiae, as they are sometimes called, 

 perform a distinct and useful function. 



The frog when developing in the water (tadpole stage) is provided with gills, but when it develops legs and 

 its swimming tail disappears, and it is fitted for a terrestrial existence, its gills are suppressed and true lungs of a 

 simple and primitive type are provided. The frog when it becomes an air-breathing animal develops characteristic 

 respiratory rhythmic movements. 



Perhaps the simplest form of lung is that met with in the newt. It consists of a long oval sac which opens by 

 a short single bronchus from a very short trachea. The walls of the sac consist of mucous membrane, epithelium, 

 connective tissue, elastic fibres, pale unstriated muscular fibres, nerves, blood-vessels, &c. The blood-vessels which 

 ramify on the sac are so placed that the blood contained in them is aerated by the oxygen contained in the air, or, 

 as happens occasionally, by the oxygen contained in the air held in solution in water. There is in the newt an 

 arrangement which admits of rhythmic muscular movements in the limg itself. 



In the salamanders, which, though air-breathing animals, are aquatic in their habits, the lungs consist of two 

 cyhndrical sacs extending nearly the entire length of the body. The air sacs have a smooth internal surface on 

 which may be traced a large number of fine capillary blood-vessels containing blood. The air is forced into the 

 lungs by a swallowing rhythmic movement and discharged at intervals to make room for a fresh supply. By this 

 simple arrangement the oxygen of the atmosphere is transferred to the blood in the capillaries of the lungs, and 

 carbonic acid extracted from it. 



The lungs of the frog are more elaborate than those of the newt and salamander because of the rudimentary dis- 

 sepiments or partitions with which they are suppUed, and which enable them to accommodate a comparatively large 

 number of capillary blood-vessels. The honeycomb structure characteristic of higher lungs makes its first appear- 

 ance in the lungs of the frog. The general structure of the lungs of the frog resembles that of the newt, inasmuch 

 as it contains as an element, pale unstriated muscular fibres capable of conferring independent rhythmic movements 

 on the lungs themselves. The frog, if it takes to the water, must, as is well known, come to the surface ever and anon 

 to breathe, and everybody is familiar with the rhythmic movements of the throat displayed on such occasions. 



It is needless to pursue the comparative anatomy of respiration further ; suffice it to say that in man there is 

 a pulmonic heart, as contra-distinguished from the systemic heart, an elaborate pair of lungs', and a comparatively 

 very large number of muscles for producing the respiratory movements by which air is taken' into and ejected from 

 the chest. (See Plate bcxvii., Figs. 1 to 8.) 



The lungs in man are composed of a larjiix, a glottis, a trachea, bronchial tubes, air cells, blood-vessels, lymphatics, 

 nerves, muscular fibres of the unstriated type, ciha, glands, epithehum, and a large quantity of elastic tissue. 



The walls of the trachea and bronchial tubes are composed of an external membrane consisting of inelastic 

 and elastic tissue and of an internal or mucous membrane. Between the membranes cartilaginous rings occur at 



