Bat body wall
Text: Sarah Winstanley and Kathy Coulombe
We removed the superficial epidermal fascia of the thorax and abdomen with small scissors and a needle point to expose the ribs and the costal muscles in the thorax and a tough aponeurotic layer just deep to the skin in the abdomen. This fascia was cut to observe the deep hypaxial muscles of the abdomen on the ventral and lateral sides. On the dorsal side, the thoraco-lumbar fascia and the dorsal hypaxial musculature were reflected to observe the epaxial vertebral musculature.
The first observation upon removing the abdominal skin dorsally was of a dark grey protrusion on the left lateral side of the abdomen through the white tough fascia. On examination, we concluded that this protrusion is an inguinal hernia. In humans, inguinal hernia occurs predominantly in men. While this hernia was an interesting find, it is understandable, since our specimen is male. The protrusion consists of 7mm in length of the intestine through the deep inguinal ring.
On the dorsal side, from superficial to deep, are the external oblique m., internal oblique m., and transversus abdominis m. The external oblique fibers run from the ribs caudally and medially, while the internal oblique fibers run at 90° to this, from the ilium in the pelvis cranially and medially. The transversus abdominis fibers run from the dorsal ribs and vertebra encompassing the cavity of the abdomen to the ventral linea alba of the rectus abdominis.
The thorax has three intercostals muscles- the external, internal, and innermost intercostal muscles. These muscles are homologous to the external oblique m., internal oblique m., and transversus abdominis m. of the abdomen, respectively. As their names imply, the intercostal muscles run from rib to rib. As in the abdomen, the fibers of the external and internal intercostal muscles lie at 90° to each other.
Dorsally, the epaxial muscles originate from the sacrum. They are called the sacrospinalis. This muscle extends cranially and separates into three distinct parts- the semispinalis, the longissimus dorsi and the iliocostalis muscles. These muscles on our specimen are very small, however they seemed to be proportioned like those of humans.
Subscapularis, pectoralis, and the posterior part of serratus dorsalis are the largest muscles in bats because they are by far the most important muscles for powering the downstroke of the wings. Attachments of the posterior part of serratus dorsalis aid in powering the downstroke, and are unique to bats as aids for flying. When the posterior portion of serratus dorsalis contracts to pull the lateral border of the scapula ventrally, the scapula rocks on its long axis, causing the medial border of the scapula to swing dorsally. One flight adaptation in bats is that the greater tuberosity of the humerus locks against the scapula at the top of the upstroke of the wing. This causes the posterior portion of serratus dorsalis to be responsible for stopping the upstroke of the wing during flight, by anchoring the lateral edge of the scapula and pulling it ventral. This action causes the locked humerus to unlock and move downward, subsequently beginning the downstroke of the wing. Thus, the serratus dorsalis in bats is relatively larger than in other mammals due to its importance during flight. [1]
| Muscle | Origin | Insertion | Action |
| Abdominal Hypaxial Musculature: | |||
| external oblique [1. p. 92] | lumbodorsal fascia and posterior ribs | linea alba, from sternum to pubis | Along with the internal oblique, compresses and supports abdominal viscera, and flexes and rotates the trunk. |
| internal oblique [1, p. 92] | lumbodorsal fascia and border of pelvic girdle | linea alba | Along with the external oblique, compresses and
supports abdominal viscera, and flexes and rotates the trunk. |
| transverse abdominis [1, p. 92] | lumbodorsal fascia and border of pelvic girdle | linea alba | compresses and supports abdominal viscera |
rectus abdominis [1,p. 92] |
pubis | sternum and costal cartilages | flexes trunk and compresses abdominal viscera |
| Thoracic Hypaxial Musculature | |||
| external intercostals [1, p. 25] | are the most superficial muscles running in between the ribs; superior attachment: inferior border of the ribsinferior attachment: superior border of ribs below | elevate ribs | |
| internal intercostals [1, p. 25] | muscles running in between the ribs, deep to the external intercostals: superior attachment: inferior border of the ribs: inferior attachment: superior border of ribs below | depress ribs | |
| innermost intercostals [1, p. 25] | are the deepest muscles running in between the ribs; superior attachment: inferior border of the ribs; inferior attachment: internal surface of costal cartilages 2-6 | probably elevate ribs | |
| serratus dorsalis: anterior part [2, p. 148-50] | a broad band along the middle sections of ribs one to five (in Myotis) | rim and anteromedial surface of flange of scapula | pulls the anteromedial border of the scapula ventrad |
| serratus dorsalis: posterior part [2, p. 150-3] | the first costal cartilage and ribs two to nine | posterior two-thirds of lateral border of scapula, lateral end of subscapularis, lateral portion of infraspinatus, and posterior cartilaginous extension of the scapula | Retracting the wing in the downstroke of flight |
| scalenes [1, p. 644] | ribs | transverse processes of cervical vertebrae | flex neck laterally, and elevate first and second ribs during forced inspiration |
| Epaxial Musculature: | |||
| multifidus [1, p. 278] | sacrum and ilium, the transverse processes of T1-T3, and articular processes of C4-C7 | spinous process, spanning segments two to four | stabilization of the vertebrae during local movements
of the vertebral column |
| splenius capitis | ligamentum nuchae and spinous process of C7-T3 or T4 vertebrae | mastoid process of temporal bone, and the lateral portion of the superior nuchal line of the occipital bone | Along with splenius cervicus, splenius capitus extends the head and neck. |
| longissimus capitis [1, p. 278, 281-2] | superior portion of the ribs, between tubercles and angles | transverse processes in thoracic and cervical regions, and the mastoid process of the temporal bone | extension and lateral bending of the atlanto-occipital joints, and ipsilateral rotation of the atlantoaxial joints;extension of the vertebral column and head: they control the motion of the flexion of the back by gradually lengthening their fibers; also lateral bending of the vertebral column |
| Remarks: This muscle is the only section of erector spinae to reach the skull. | |||
1. Agur, A.M.R., and Lee, M.J. 1999. Grant's Atlas of Anatomy-10th edition. Lippincott Williams and Wilkins, New York.
2. Vaughan, T.A. 1970. "The Muscular System", p. 139-194 in Wimsatt,
W.A. (ed.) Biology of Bats: Vol. 1.Academic Press, New York.