CIRCULATION. 



679 



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.f 



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, which divides at once into more than 

 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. J 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 fetal condition of the arteries in which 



' J. Hunter. 



t Miiller's Physiol, vol. i. p. 198. 



\ See the accounts of these varieties by J. Hun- 

 ter in the Phil. Trans. Sharpey, Meeting of 

 British Scient. Assoc. in Edin. Sept. Ib34. Breschet, 

 Annal. des Scien. Natur. 1834. Baer, Nov. Act. 

 Nat. rur. 1835. 



$ Cuvier, Lemons d'Anat. Comp. vol. iv. 



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.f 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 ;J 2, security against obstruction 

 of the circulation from pressure, as in climbing 

 animals which cling long and forcibly 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 Cetacea. || 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 

 may 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 muscularity of 

 these sinuses alleged by some must have the 

 effect of emptying them more easily than would 

 be accomplished by the vis a tergo. 



4. Influence of the nervous system upon 

 the circulation. It is a very general opi- 

 nion among physiologists that a considerable 

 influence is exerted by various parts of the 

 nervous system upon the function of circu- 

 lation as a whole, and through it upon the 

 different processes of the economy concerned 

 with nutrition, as digestion, secretion, growth, 

 animal heat, &c. There is some difficulty, 

 however, in ascertaining the exact relation 

 which subsists between particular parts of the 

 nervous and circulatory systems. It is mani- 

 fest that in many instances the circulation in 

 the bloodvessels is modified by a nervous in- 

 fluence which operates on the heart alone, whiJe 

 in others it is affected by an alteration of the 

 vital powers of the bloodvessels themselves. 

 We refer the reader to the articles CONTRAC- 

 TILITY and HEART for an account of the 

 modifications to which the circulation is liable 

 from the operation of nervous influence on 



* Baer, loc. cit. 



f Barclay on the Arteries, p. 36. 

 t Carlisle, Phil. Trans. 1800. Roget, Bridge- 

 water Treatise. 

 Vrolik. 

 J. Hunter, loc. cit. 



