In the beaver the internal carotid is larger than the vertebral.
In the porcupine the internal carotid, after following for some distance the direction of the internal maxillary,' vvithout undergoing any sinuous flexures, enters the cranium through the foramen lacerum anterius, where it immediately joins with the basilar, which surpasses it in size, to form the circle of Willis.
In the Guinea-pig and the agouti there is, properly speaking, only an external carotid, of which the internal carotid is but a small branch. This little cerebral branch is derived from the internal maxillary, of which it seems to be a continuation ; it enters the cranium through the foramen ovale of the sphenoid bone, and joins the circle of Willis, which is here principally formed by the vertebral artery.
In the squirrel the internal carotid enters an osseous canal in the tympanum, through the jugular foramen, passes between the crura of the stapes, and then penetrates the cra nium through a hole in the petrous portion of the temporal bone ; it there divides into two branches, the smaller of which enters a deep groove in the os petrosum, issues from the cranium through the foramen lacerum an terius to enter it again through the oval foramen of the sphenoid bone. It .is only after all these windings that it divides into small branches, and of these only one or two go to form the circle of Willis, the rest being meningeal arteries. The continuation of this branch subsequently becomes the representa tive of a portion of the ophthalmic artery. The other branches usually given off from the ophthalmic artery are derived from the second branch of the internal carotid above mention ed, which previously gives off branches to the dura mater. It will thus be seen that the in ternal carotid supplies very little blood to the brain, and this blood only arrives at its desti nation by a very circuitous route. t In the marmot the internal carotid at first follows the same course as in the squirrel ; it enters the canal of the tympanum through the jugular foramen, and then traverses the opening between the crura of the stapes, after which it divides into two branches : of these the internal, which is the smallest, runs through an ascending canal, which enters the cavity of the skull close to the sella tureica, arriving at the brain much in the same manner as tile 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 ulna: 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 conununicates with the external jugular, carrying off its share of the blood limn the interior of the cranium.