NEW VIEW OF THE MECHANISN OF RESPIRATION 285 



muscular arrangements are only equalled by its numerous, complicated, and extensive attachments. No muscle 

 in the body has more complex surroundings. It is a purely involuntary muscle, and yet its muscular fibres have the 

 transverse strise or markings of the voluntary muscles. It is an original fundamental structure : in other words, 

 it is formed in utero before it is called upon to perform any function. It is only after birth, and after the first breath 

 is drawn, that it asserts itself as a working unit of the animal economy. It is a spontaneous, self-acting, rhythmic 

 muscle, and is not dependent on irritabihty, elasticity, or extraneous stimulation of any kind for its movements. 

 It is not forced into activity by the resihency of the lungs or ribs : neither is it affected to any extent by pressure 

 exerted on it by the viscera (stomach, Uver, intestine, &c.). While it works in harmony with all the respiratory 

 muscles it is guided by none. It exerts a centripetal and centrifugal power and exercises a pulling and a pushing 

 force. It pulls in inspiration and pushes in expiration ; the pull, because of the flaccidity of the muscle, being more 

 marked than the push. 



The movements of the diaphragm have been already described. 



The deeper muscles of the chest and abdomen (if the recti abdominis and the external oblique abdominal 

 muscles be excepted) show the mechanism of respiration best. What is here stated will be readily understood by 

 a study of Plate Ixxviii., and of Figs. 50 and 51, where the thoracic and abdominal muscles are accurately given. 

 It is necessary, however, to be acquainted with both the superficial and deep muscles of the chest and abdomen, as 

 these two kinds of muscles are co-ordinated and act in xinison ; the chest muscles closing or contracting when the 

 abdominal muscles open or dilate, and vice versd. 



The superficial and deep muscles act synchronously or together, so that all the muscles of the chest and all those 

 of the abdomen are in a state of constant motion. In other words, the superficial and deep sets of thoracic and 

 abdominal muscles are at work at the same time. Finally, the chest and abdominal muscles, regarded as wholes or 

 aggregates, are all moving at once. This arrangement provides for the alternate opening and closing of the thorax 

 and abdomen during the entire hfe of the individual, both in the waking and sleeping states. There are corresponding 

 synchronous movements in the nostrils and glottis, and in the diaphragm. 



The respiratory movements are at once extensive and comphcated, and have not hitherto been satisfactorily 

 explained. The movements in question are strictly rhythmic in character. They occur in striated muscles, the 

 striated muscles, as a rule, being voluntary muscles. There is thus a departure from use and wont which strongly 

 supports my contention that the involuntary muscles are the precursors and, in a sense, the parents of the voluntary 

 ones, and that the movements of even the voluntary muscles are essentially rhythmic in their nature.^ 



The chest muscles consist of the external and internal intercostals, the pectoralis minor, the serratus magnus, 

 and the pyramidalis sterni. All these muscles are engaged in elevating the ribs. The action of the glottis and 

 diaphragm is not now considered. The abdominal muscles which pull the ribs downwards are the recti abdominis 

 and the external and internal oblique abdominal muscles. The transversales abdominis are powerful expiratory 

 muscles situated beneath the external and internal oblique ones. Their muscular fibres run at right angles to those 

 of the recti abdominis muscles. The recti abdominis muscles are provided with a fibrous sheath derived from the 

 external and internal and transversalis abdominis aponeurosis and display tendinous transverse markings (lineae 

 transversse), which some are disposed to regard as elementary ribs. The points here raised are illustrated at 

 Plate Ixxviii. and Figs. 50, 51, 52, and 53. 



PLATE LXXVIII 



Plate Ixxviii. displays the thoracic and abdominal muscles engaged in respiration. It shows that the respiratory 

 movements are not due to movements occurring in the chest and diaphragm alone, but to combined co-ordinated 

 and alternate movements, which occur simultaneously in the thoracic and abdominal muscles, and in the diaphragm, 

 respectively. This view of the respiratory muscles and movements, although novel, appears to me to meet all the 

 requirements of the case. 



Fie. I.— a, a', External intercostal muscles (human) ; b, V, internal intercostal muscles. The e.\ternal and internal intercostal muscles 

 when they contract ruck up the ribs and force the sternum or breast-bone outwards, c, Pectoralis minor ; this muscle is a powerful 

 elevator of the ribs. The ribs are pulled down by the contraction of the rectus abdominal muscles aided by the external and internal 

 oblique muscles, d, d', Serratus magnus muscle ; this is an elevator of the ribs, e, Subscapularis muscle ; /, teres major ; g, latissimus 

 dorsi ; these three muscles rotate the humerus in the direction of its length, h, Biceps niusole ; this ilexes the forearm on the arm. 

 i, Clavicle ; j, pectoral muscle cut across. The numerals 1 to 8 inclusive indicate the first eight ribs. 



Fig. 2.— In this figure the arms are raised, a, a', Linea alba or white fibrous mesial line ; '), h', semi-lunar fibrous line ; c, c', rectus 

 abdominis muscle (of tfiese there are two— one on either side of the white line or linea alba). The rectus abdominis is covered by its 

 sheath and partly separated by tendinous intersections (linex transversal). The recti muscles play an important part in expiration by 



1 Examples of mixed muscles where the movements are partly voluntary and partly involuutary are found in the fauces. 



