ANATOMY OP BA LiENOPTERA ROSTRATA. 
221 
over the thorax, was connected below and behind with the external oblique and rectus 
abdominis muscles posteriorly, and appeared to be the combined equivalent of the 
scalenus medius, rectus sternalis, and supracostalis muscles. The most external or 
superficial fibres arose from the posterior portion of the three anterior ribs. The fibres 
of these three portions converged, and uniting formed a single inseparable musculoten- 
dinous mass which was inserted into the basilar process of the occipital bone extending 
as far forwards as the posterior orifices of the nares, and by some tendinous slips into 
the inferior and anterior cervical transverse processes. 
On removing this muscular mass a very large plexus of blood-vessels, principally 
venous, was seen extending from the first cervical to the fourth dorsal vertebra, lying 
on the heads of the ribs, and passing also upwards into the large spaces formed by the 
transverse processes of the cervical vertebrae ; a portion of this plexus likewise passed 
in the interval between the two heads of the trachelomastoid muscle, and in the cup- 
like cavities or interspaces between the necks of the ribs. In the interval between the 
transverse processes of the second and third dorsal vertebrae it received a large vein 
which passed out of the spinal canal ; this vessel and its fellow of the opposite side were 
traceable upwards to the interior of the cranial cavity, one lying on each side of the 
medulla spinalis external to the dura mater, and its origin appeared to be from a venous 
sinus formed within the cranial dura mater. Arteries derived from the internal carotid 
accompanied these veins, and were arranged in a somewhat similar, though more 
numerous, finer, and more intricate retiform manner. 
At the posterior or thoracic part of this rete mirabile the veins gradually coalesced, 
and thus formed several large trunks on each side of the spine ; these again uniting 
ultimately formed two large veins, which were distinctly traceable into the superior vena 
cava immediately prior to its entrance into the right auricle. Two large spinal arteries, 
after traversing this rete, were traceable downwards along the spinal canal, one on each 
side of the medulla spinalis on the external surface of the arachnoid, but within the 
dura mater, gradually converging and becoming smaller in calibre until they reached a 
point opposite to the articulation of the first or most anterior chevron bone, where they 
united and formed a single vessel, which was from thence traceable to near the termi- 
nation of the spinal canal in the vertebrae of the tail. These arteries were accompanied 
anteriorly by a somewhat similar arrangement of veins, branches passing into the internal 
jugular, which were perhaps less complex in their anastomosis than were the arteries, 
although the latter constituted a larger part of the plexuses. 
The Depressor maxillae inferioris, situated behind the angle of the jaw, which it clothed 
and almost completely concealed, was a powerful deltoid-shaped muscle, and arose from 
the posterior surface of the mastoid process of the squamous bone, and the sulcus 
behind and internal to it ; the fibres converging, ran downwards and forwards to be 
inserted into the posterior and inferior surface of the angle of the maxilla, and also 
into a portion of its ramus for the extent of about 4 inches anterior to that point. This 
muscle appeared to be homologous with the digastric of other mammals, from its posi» 
