Clinical Anatomy of the European Hamster 
spends to the fourth rib dorsally. The second rib 
curves somewhat caudally in its dorsal aspect and 
somewhat cranioventrally in its ventral aspect. 
Therefore, the ventral part is markedly wider than 
the dorsal part of the first intercostal space (spati- 
um intercostale), which is the broadest of the 12 
spaces. The remaining ribs run caudoventrally from 
their dorsal origin to the manubrium and bend 
somewhat cranial in the cartilaginous parts ven- 
trally (Figs. 4-3, 4-4). The second to sixth ribs end 
at the cartilaginous discs between the sternebrae. 
The seventh rib pair does not reach the sternum but 
fuses with the cartilaginous ends of the sixth rib 
pair (Fig. 4-1). The twelfth and thirteenth ribs are 
floating (Figs. 4-1, 4-3). 
The thoracic vertebrae form a kyphosis, whose 
vertex lies between the thirteenth thoracic and first 
lumbar vertebrae and which is continued into the 
lumbar and sacral parts of the vertebral column 
(Figs. 3-1,4-1). The length of the thoracic column 
is 63.3 mm, nearly three times that of the cervical 
column (Fig. 3-1). The axial thoracic skeleton con- 
sists of thirteen vertebrae (vertebrae thoracicae) 
whose length and breadth increase caudally (Figs. 
4-1, 4-2). The second thoracic vertebra has the 
tallest spine {processus spmosus), 9.5 mm high 
(Fig. 4-8). The spinal processes of the first nine 
thoracic vertebrae are caudally oriented, each over- 
hanging the body of the next vertebra, while the 
remaining four lie in a cranial or perpendicular 
direction and do not overhang adjacent vertebrae 
(Fig. 3-1). The first vertebral bodies are dorso- 
ventrally compressed (Fig. 3-1). With increasing 
vertebral number, the transverse section becomes 
more semi-circular. 
Up to the ninth or tenth vertebra, the surface of 
each articular process (processus articularis) lie 
almost horizontally permitting lateral as well as 
dorsoventral movement. 
4.2 THORACIC MUSCULATURE 
The pectoralis muscle (m. pectoralis), which 
connects the sternal region with the humerus and 
forms a triangular surface between the clavicles, 
xiphoid process and humerus (Fig. 3-7), is divided 
into a superficial pectoral (m. pectoralis superfi- 
cialis) and a deep pectoral (m. pectoralis profun- 
dus). There is a cranial descending part (pars 
descendens) and a caudal transverse part (pars 
transversus) of the superficial pectoral (Figs. 3-12), 
The deep pectoral is overlapped by the superficial 
pectoral. Since the caudal part of the latter is very 
thin, it is difficult to separate the two muscles at this 
point. The intercostal muscles, which span the in- 
tercostal spaces, are subdivided into internal and 
external divisions. The external intercostals (mm. 
intercostales extemi), which regulate inspiration, 
originate on the caudal margin of the cranial rib of 
each intercostal space and run caudoventrally to the 
cranial margin of the caudal rib. The internal inter- 
costals (mm. intercostales interni), which regulate 
expiration, originate on the cranial edge of the cau- 
dal rib of the intercostal space and run cranioven- 
trally to the caudal margin of the cranial rib. The 
mm. levatores costarum represent a vertebral rein- 
forcement of the external intercostals, from which 
they can not be completely separated. The levators 
originate on the transverse processes of the first to 
twelfth thoracic vertebrae and run caudoventrally 
to the angle (angulus costae) of each succeeding 
caudal rib, where they insert on the cranial margins. 
4.3 THE DIAPHRAGM 
The diaphragm (Figs. 4-9, 4-10, 4-11), which 
curves from the twelfth rib to the xiphoid process 
(Fig. 4-3), has a horseshoe-shaped central tendon 
(centrum tendineum) and a periphery consisting of 
strong muscle fibers (pars muscularis), grouped 
into three parts: sternal, costal and lumbar. The 
muscle fibers of sternal origin are affixed to the 
ventral side of the second lumbar vertebra cranial 
to the curvature of the last costal pair and ventral to 
the dorsal surface of the xiphoid process. The lum- 
bar muscle fibers are composed of the left and right 
lumbocostal arches (arcua lumboco stales), which 
originate at the third lumbar vertebra (vertebra 
lumbalis). The crura are not sharply differentiated, 
as in larger mammals. The esophageal aperture 
(hiatus oesophageus), which lies at the level of the 
eighth or ninth rib, is located in the muscular part 
of the diaphragm dorsal to the crural bifurcation of 
the central tendon. Dorsal to the esophageal hiatus 
and close to the vertebral column is the aponeurotic 
opening for the descending aorta (hiatus aorticus), 
found in the muscular part between the left and 
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