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296 DESIGN IN NATURE
Expiratory Muscles^ Engaged in Ordinary Calm Breathing
Attachments.
(/) The recti abdominis muscles. The rectus abdominis arises by two tendons from the crest of the os
pubis and the ligiiment covering the symphysis pubis. It is inserted
by three portions into the fifth, sixth, and seventh ribs anteriorly.
((/) The external, internal, and transversalis The external oblique nuiscle is attached to the external surfaces and
abdominis muscles (especially the two
former).
(/() The triangularis sterni.
(i) The serrati muscles.
(j) The sacro-lumbalis muscle.
loMer borders of the eiglit inferior ribs—crest of the iliiini, linea alba,
and Poupart's ligament. The internal oblique muscle is attached to
Poupart's ligament, anterior portion of the crest of the ilium and
lumbar fascia— the cartilages of the four inferior ribi=, the linea alba,
the upper crest of the pubis and the pectineal line. The transversalis
abdominis is attached to Poupart's ligament, anterior portion of the
crest of the ilium, the lumbar vertebrae, the inner surfaces of the
cartilages of the six lower ribs—the crest of the pubis, pectineal line,
aiid linea alba.
The triangulai'is sterni is attached to tlie ensiform cartilage, lower
border of sternum, and the lower four costal cartilages—the cartilages
of the second, third, fourth, and fifth ribs.
The serratus muscle (divided into superior, middle, and inferior portions)
arises by eight fleshy digitations from the external borders and
upper surfaces of the eight uppei' ribs and the aponeurosis covering
the upper intercostal spaces. It is inserted into the whole length of
the inner margin (if the posterior border of the scapula.
The sacro-lumbalis is attached to the sternum—the angles of the six
lower ribs.
(/c) The diaphragm. This muscle takes part in The diaphragm. For attachments of this muscle, see description given
expiration and inspiration. under (e).
While the above table illustrates very satisfactorily the nuniber and nature of the muscles engaged in calm
normal breathing, it is well to bear in mind that a comparatively large number of extraordinary or auxiliary muscles
are brought into action in deep or forced breathing. The movements of the auxiliary muscles are by no means well
determined ; a circumstance due to the auxiliary muscles, in many cases, acting singly or in small groups, and
to their being, in not a few instances, removed to considerable distances from the thorax and abdomen. After all,
it is with the ordinary muscles of inspiration and expiration we have to do. It will suffice simply to enumerate the
extraordinary or auxiliary muscles of respiration. They are briefly stated as under by Professors Landois and
Stirling :
'^
Muscles engaged in Forced Inspiration in the Trunk.—The scaleni, the sterno-cleido-mastoidei, the trapezii, the
pectorales minor, the serrati, the rhomboidei, and the extensors of the vertebral colunm.
Muscles en-gaged in Forced Expiration in the Trunk.—The abdominal muscles, the intercostal nmscles, the triangularis
sterni, the serratus posticus inferior, and the quadratus lumborum.
It should be noted that in addition to the respiratory muscles enumerated above, others occur on the nostrils,
and on the pharjnax and larynx ; the latter regulating, within limits, the movements of the vocal chords in the
production of voice, and the ingress and egress of air into the lungs.
It only remains to say a few words regarding the causes which produce the muscular rhythmic movements of
respiration. Are they due to inherent irritability and stimulation and elastic recoil acting on the tissues involved ;
or are they the outcome of original endowment stamped on the animal from the beginning and pre-determined to act
spontaneously and as apart from elasticity, irritabiUty, and stimulation, and also as apart from nerves, which are
rather to be regarded as the regulators than the originators of the movements ?
For reasons already stated I am strongly of opinion that the muscular, respiratory, rhythmic movements are spon-
taneous and independent movements, and that they are conferred on animals as part of their original equipment,
without which the business of hfe could neither be inaugurated nor continued. One has only to recollect that so
long as life endures the demand for breath is incessant day and night, and that the movements of the muscles of
respiration are specially adapted to the performance of one great function, namely, the supplying of the lungs with
pure air. If the supply of pure air be not forthcoming speedy collapse inevitably follows. To keep the air in the
lungs pure there must be no hitch or impediment in the due performance of the breathing process. There must
be no waiting for elastic recoil at any stage of the respiratory act.
^ These nmscles, as already explained, also take part in inspiration.
^ "A Text-Book on Human Physiology, including Histology and Microscopical Anatomy,'' by Dr. L. Landois, Professor of Physiology,
University of Greifswald, and William Stirling, M.D., Sc.D., Professor of Physiology, Owens College, University of Manchester.