. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. RODENTIA. 391 through an ascending canal, which enters the cavit} of the skull close to the sella turcica, arriving at the brain much in the same manner as the internal carotid of the human subject. This branch is smaller than the vertebral artery. The other or external branch enters the cranium through a canal that opens upon the anterior surface of the petrous bone, and divides into the middle meningeal and ophthal- mic arteries. In the dormouse the distribution of the internal carotid very nearly resembles what is des
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. RODENTIA. 391 through an ascending canal, which enters the cavit} of the skull close to the sella turcica, arriving at the brain much in the same manner as the internal carotid of the human subject. This branch is smaller than the vertebral artery. The other or external branch enters the cranium through a canal that opens upon the anterior surface of the petrous bone, and divides into the middle meningeal and ophthal- mic arteries. In the dormouse the distribution of the internal carotid very nearly resembles what is described above, as occurring in the squirrel and in the marmot. In some genera of Rodents the internal condyle of the os humeri is perforated by a canal through which the ulnar artery passes in company with the median nerve : this arrangement exists in the squirrel, the hamster, and the helamys. Venous system. — In most of the Rodentia, instead of a single anterior vena cava, there are two principal anterior trunks of the venous system, one of which, namely the right, occu- pies the usual position of the vena cava anterior, whilst the left runs along the furrow that separates the base of the ventricle of the heart from the left auricle, to reach the right auricle, into the upper and left side of which it opens. In those genera which hibernate the exter- nal jugular vein likewise presents a very remarkable arrangement. This vein receives a considerable proportion of the blood de- rived from the brain through a wide canal, situated between the os petrosum and the temporal bone, into which the anterior division of the transverse sinus opens, so that it is only the smallest moiety of blood derived from the vein which escapes through the jugular for- amen into the internal jugular. The vertebral vein likewise communicates with the external jugular, carrying off its share of the blood from the interior of the cranium. Fig. allta Upper surface of the liraln of the Porcupine. (A
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