Picture - Cosmic Polymath

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