274 DESIGN IN NATURE 



or automatically there is a reversion to the primitive type of movement as it occurs in the respiratory, circulatory, 

 ahmentary, and reproductive rhythms. Whether movements are voluntary or involuntary is largely a question 

 of habit, repetition, and training, in which the nervous system plays a not unimportant part. 



The muscular fibres of the stomach, bladder, and uterus are of the pale unstriated order. The movements 

 of these viscera are strictly involuntary ; but (and this is remarkable) the heart, which is a typical involuntary muscle 

 as regards its rhythmic movements, is composed of red striated muscular fibres. In the case of the respiratory muscles 

 they are partly of one land and partly of the other. Thus the muscular fibres investing the trachea and bronchial 

 tubes are of the pale unstriated sort, whereas those of the diaphragm, the thorax, and the abdomen and other 

 muscles concerned are of the red striated, voluntary kind. The respiratory movements produced partly by pale 

 unstriated and partly by red striated muscles are, however, typical rhythmic movements over which, as explained, 

 we have the slenderest possible control, and only for a very limited period. In other words, rhythmic movements 

 are not peculiar to either kind of muscle ; a fact which in itself goes far to break down the arbitrary but convenient 

 distinction between the voluntary and involuntary system of muscles. (The pecuharities of the voluntary and 

 involuntary muscles, and their precise mode of action, are illustrated at Plate Ixxxiii., p. .320.) 



These prehminary observations are necessary in speaking of the respiratory rhythmic movements, as much 

 ignorance and misapprehension prevails as to their precise nature and the number of the hard and soft parts, 

 especially muscles, which take part in them. 



The object of respiration in every instance is the interchange of gases. In plants and in the lowest animal 

 forms that interchange takes place in a very simple way, and without the intervention of any special apparatus. 

 The plant is enveloped in air, and air mingles and circulates with its juices throughout its entire substance. During 

 the day the plant takes in the carbonic acid of the atmosphere and gives out oxygen. 



The amoeba, which may be taken as an example of the lowest animals, is similarly circumstanced. It is invested 

 by air or air and water, and being molecular and largely structureless the air or the oxygen of the air finds easy 

 passage through its substance. It takes in oxygen and gives out carbonic acid. The same holds true of the skin and 

 mucous Knings and all the tissues of the body even in the highest animals. It is only when hearts and a system 

 of blood-vessels containing blood or nutritious juices make their appearance that distinct respiratory organs become 

 necessary. In the higher animals (bird and mammal) the connection between the respiratory organs and the circulation 

 becomes very evident ; the lungs being provided with a heart for themselves, known as the right or pulmonic 

 heart, as contra-distinguished from the left or systemic heart— the right heart forcing the blood through the lungs, 

 the left heart forcing it through the system generally. 



§ 55. The Respiratory Organs in Animals and in Man Structurally Considered. 



The respiratory organs are called gills and lungs respectively. They are designated gills in the fish, the water 

 hzards, &c., and lungs in the reptile, bird, and mammal. 



PLATE LXXVII 



Plate Ixxvii. illustrates the structure and position of the human limg within the thorax : also the narrow chink 

 or sht m the larynx through which the air passes in respiration, and the position and mode of action of the vocal 

 chords. It also shows the structure of the lung of the fish, menobranchus and too 



Fig. 2.— Larynx, glottis, trachea, bronchial tubes, and air cells of human lung (after Dalton) 



frog j::^^-^: ^« -^^?s,n^s;-r3r^> - 1-^^ -r^ - - 



are s^'^^A^i z ^ziz^:ia^:i:,^^:zST ^" '''- ^"^'^^-^ '-^'- ^^^ ^^"^ -« ^^^^ ^^ --^-- -^ 



diaiAra™' liv.^r°*?,Kl?f f" antero-posterior section of a frozen human cadaver seen from behind. Shows the lun^, hem-t 

 Tln?v^«^„?„' ^"'"'iT^ stomach in their natural positions ; the thorax and abdomen being separated from each other In tl, "^^ 'i, ' 



J he viscera completely occupy the thoracic and abdominal cavities. The heart is situated in the midXe of the thomx and h^ T"' 



