Bauhaus-Universität Weimar

242 
VASCULAR PLEXUSES. 
Arterial and venous plexuses, or relia mirabilia. 
These vascular plexuses are, without doubt, to be reckoned among 
the facts in comparative anatomy which are of great physiological 
interest. They are structures formed by the sudden division of an 
artery or vein into the tuft of smaller twigs, or into numerous anas¬ 
tomosing branches, which may or may not unite again into a single 
trunk. The cases in which this reunion takes place are the more 
rare. It is met with in the arteries of the extremities and tail of some 
tardigrade Mammalia, such as Bradypus, Myrmecophaga, Manis, 
and Stenops.* The same structure occurs, however, in parts where 
it can have no relation to muscular motion; thus the rete mirabile 
caroticum in the Ruminantia and hog is formed by branches of the 
common carotid which again unite to form the cerebral carotid. 
Rappt shows, that in animals provided with a rete mirabile caro¬ 
ticum, the vertebral artery does not go to the brain, but is connected 
with the external carotid, as in the goat and calf, or where it has 
connections with the rete mirabile of the carotid, is still principally 
distributed to the muscles of the neck, as in the sheep. Similar 
plexuses of arteries exist in the orbits of Ruminantia, feline animals, 
and birds, according to Rapp and Barkow,± and give origin to the 
arteries of the eyeball. The vascular plexuses connected with the 
intercostal arteries and iliac veins of the Delphinæ are of immense 
size.§ In some birds the arteria tibialis forms a rete mirabile. 
Among reptiles and Amphibia we know only the small vascular 
plexus seated on the trunk of the carotid of the frog, the so-named 
carotid gland. Some of the largest structures of this kind are those 
recently discovered by Eschricht and myself in certain fishes; and 
they are the more remarkable for being composed both of arteries 
and veins. 
These vascular structures have either a mechanical or a chemical 
influence on the blood. The transmission of the blood through nu¬ 
merous small tubes which unite again to form single trunks, appears 
to have the object of producing a local retardation of the circulation 
by the increased friction to which it gives rise. This view is appli¬ 
cable to all forms of rete mirabile. The theory that they are destined 
to secure the continuance ot the circulation in the extremities and 
tail of some climbing animals during prolonged action of the muscles, 
will apply to a few forms only. There are at present no other grounds 
for supposing that these vascular plexuses have a chemical action 
than their analogy with the plexuses of lymphatics and the lymphatic 
glands, and the observation of Dr. J. Davy, that the thunny has the 
power of maintaining a temperature much above that of the sur- 
* Sir A. Carlisle, Philos. Transact. 1800. Vrolik, De peculiari Art. Extremi- 
tatum in nonnullis Animalibus Dispositione. Amsterd. 1826. 
f Meckel’s Archiv. 1827. 
X Meckel’s Archiv. 1829. 
§ Breschet, Hist. Anat. et Physiol, d’un Organe de Nature vasculaire décou¬ 
verte dans les Cétacés. Paris, 1836.—Baer, Nov. Act. xvii. These vascular 
plexuses were known to Tyson (Anatomy of a Porpoise), and were more accu¬ 
rately described by Mr. Hunter (On the Structure and Economy of Whales).
        

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