3. Iqfluence of' varieties in the distribution of arteries and veins upon the circulation.—As connected with some of the above-mentioned facts, and exerting a considerable influence in modifying the circulation of the blood in parti cular states of the animal economy, we may here mention a few of the more remarkable varieties in the distribution of the arteries and veins, together with the uses they have been supposed to serve in different animals. The varieties of form in the larger arteries may be considered under two heads ; a, simple tor tuosity ; and b, sudden division into many small branches.
a. One of the best examples of the first of these varieties, which are by no means uncommon in animals, occurs in the spermatic arteries of the bull. Two reasons have been assigned for the existence of this, viz. 1, to allow, by the greater length of the vessel, for the stretching of parts, as in the arteries of the lips ; and 2, to dimi nish the velocity of the blood passing through the tortuous vessel, from the longer course and greater incurvation.* Increased friction, which must be the consequence of greater length of the artery, will diminish the velocity of the blood through the whole vessel, and besides this, a given particle of blood passing through a tortuous vessel will arrive later at its destina tion, in consequence of the longer course it has to run through; but if we regard the fluid in the arteries as every where subjected to pres sure, it is very doubtful that the increased cur vature can be the source of any considerable retardation by diminishing the force communi cated by the impulses of the heart.t b. The sudden division of an artery into many small branches may take place with or without tortuosity or a plexiform arrangement ; the primitive vessel disappearing or persisting, but in most cases when present, diminished in size. The most remarkable examples of this peculiarity of the arterial system are the follow ing. 1. The intercostal and lumbar arteries of the Cetacea in the posterior part of the chest, and in the vertebral canal and the caudal artery of the same animals, which are tortuous and plexiform. 2. The brachial artery of the Por poise, i.vhich divides at once into more tInn forty plexiform branches. The primitive trunks disappear, and five or more vessels emerge from the distal end of the plexus. The uterine and vesical arteries of the same animal are much divided, but not plexiform.t 3. The subdi vided brachial and crural arteries of the Bra dypus tridactylus, Lemur tardigradus, L. gracilis and L. tarsius ; and the same arteries, as well as the caudal arteries of the Myrme cophaga didactyla and M. tetradactyla. 4.
The arteries of the legs of the Swan, Goose, and Turkey divide into several long branches, which anastomose with one another.§ 5. The rete mirabile of Galen on the internal carotid of many quadrupeds, and the rete mirabile on the common carotid of the Frog. 6. The rete mirabile of Hovius on the ophthalmic artery of some animals, the Seal for instance. 7. The mesenteric arteries of the Sow at their com mencement. 8. The subcutaneous arteries of the Hedgehog.
The uses of these very various forms of arte ries it must be confessed is very little known. Some of them may, like other peculiarities in animal structure, and more especially those be longing to the vascular system, be remains of the fcetal condition of the arteries in which they exist.* The most common opinion enter tained as to their effect on the circulation is that they retard the velocity of the blood, and render its flow more uniform, thus preventing the parts supplied by them from being affected by sudden changes.t Other secondary conse quences of the diminished velocity occasioned by these peculiar structures have been imagined, as for example, 1, diminished rapidity and greater durability of muscular contraction, as in the Sloths; I 2, security against obstruction of the circulation from pressure, as in climbing animals which cling long and thrcibly to branches of trees; § 3, or these plexuses have been regard ed as intended to increase the capacity of the arterial system, and to serve as reservoirs for blood, as may be the case in the Cetaceall In some of the above-mentioned animals the tor tuosity or multiplied divisions of the arteries are accompanied by a similar condition of the veins, as in the Porpoise.
The most remarkable variety in the form of the venous system, and the one to which a use rnay be most easily assigned, is the large dila tation of the vena cava inferior in the neigh bourhood of the liver, which occurs in those animals which from their mode of life are in the habit of remaining long under water, such as the Seal, Otter, and Diving Birds. The pur pose of the venous sinuses in these situations is manifestly to allow of the accumulation of venous blood in the vena cava without an un usual distension of the right side of the heart and bloodvessels leading into it and from it, which is the effect of long submersion or im peded respiration in animals unprovided with this peculiarity of structure. The venous and arterial plexuses of the Cetacea very probably serve the same purpose. The nmscularity of these sinuses alleged by some must have the effect of emptying them more easily than would be accomplished by the vis tergo.