74 STUDIES IN COMPAKATIVE ANATOMY. 



Van der Kolk and Yrolik,^ Hyrtl,^ and others ^ have collected 

 many examples of vascular plexuses in vertebrate animals, and 

 have suggested several possible theories of their physiological 

 effect. It seems necessary to distinguish and classify before aDy 

 explanation is attempted, for no single explanation can possibly 

 apply to all the recorded cases. Arterial and venous plexuses 

 must be carefully separated, though this has not always been 

 attended to. With respect to the arrangement of their com- 

 ponent vessels, some plexuses arise by the breaking up of a 

 trunk into many small branches, which subsequently reunite, 

 and may be compared to a rope untwisted in the middle {/uni- 

 form plexuses). Others form a network of communication be- 

 tween different vessels {retiform plexuses). Others, again, are 

 due to the sudden breaking-up of an artery into many small 

 branches, which may anastomose, but do not reunite ; or to the 

 entry of many small veins into a common trunk at or near the 

 same place (distributive plexuses). 



Such plexuses as we have seen in the elephant are all venous 

 and retiform. We have no confident opinion respecting their 

 physiological meaning. It may be worth while to remark that 

 they are so extensive as to constitute in the aggregate a not 

 inconsiderable reservoir of blood, which may remain nearly 

 motionless, or move sluggishly when the animal is at rest, but 

 must be impelled towards the heart by energetic contractions of 

 the adjacent muscles. The small development of the lymphatic 

 system is also an important, and not impossibly a related fact. 

 The lymphatic glands are few and small, and the thoracic duct 

 was not, in our example, materially larger than in an adult man. 

 That the venous blood lodged in these many plexuses may have 

 an absorbent office is a possible view, but one upon which we do 

 not venture to lay emphasis. 



^ " Recherches surles plexus vasculaires chez differentsanimaux," Zool. Soc. of 

 Amsterdam, transl. in Ann. Sci. Nat. Zool. 4^ ser. tom. v. p. Ill (1856). 



2 " Neue Wundernetze und Gefleclite bei Vbgeln und Saugethieren, " Kais. Akad. 

 d. Wiss. Math, naturiv. Bd. xxii. (1863). Also reprinted separately, 1864. 



^ Miiller, Von Baer, Carlisle, Breschet, Mibie Edwards, Turner, Murie, &c. 



