Mar. 12, 1874] 



NATURE 



369 



been so, that in animals with a heart so imperfectly 

 divided, the blood sent to the lungs would be necessarily 

 a mixture of venous and arterial lluid, and similarly that 

 the blood distributed by it to all the organs and parts of 

 the body is alike a mixture of pure and impure fluid. 



In fact, however, this is by no means the case, and in 

 the frog, in spite of the reception into a single chamber 

 of both venous blood from the body, and of arterial blood 

 from the lungs, special mechanical arrangements effect 

 such a definite distribution of the two sorts of blood, that 

 the unoxygenated fluid from the body is sent to the 

 purifying respiratory surfaces (lungs and skin), and that 

 the pure oxygenated blood alone goes to the head and to 

 the brain. 



For the detection of this beautiful mechanism, we are 

 indebted to the careful investigations of Ernst Rriicke.* 



The heart of the frog consists of a right and left auricle 

 (divided by a delicate septum), both opening into a single 

 ventricle. From the latter proceeds an aortic root (bulbus 

 aorta:) which gives rise to three arterial trunks on each 

 side. 



The first of these, or carotid trunk (i), ends in 

 an enlargement (rz) tenned the carotid gland, of spongy 

 structure, which gives rise to two arteries, one the lingual 

 (/), the other {c), the carotid which goes to the head and 

 brain. 



The second, or systemic trunk (2) meets its fellow of 

 the opposite side beneath the spine, and thence passes 

 backwards as the great dorsal (in man descending) aorta, 

 giving off arteries to all parts ot the body. 



The third, or pulmo-cutaneous trunk (3) ends by 

 dividing into two arteries. The anterior of these {r) goes 

 to the skin (which, as we have seen, is in the Frog an 

 important agent in respiration), the posterior one i^p) goes 

 to the lungs. 



The heart itself is of a more or less spongy texture, but 

 the main cavity of the single ventricles open at its extreme 

 right into that of the aortic bulb (<:). In close proximity 

 to the opening are the openings from the right (Jb) and the 

 left {a) auricles respectively. 



The aortic bulb is constitutionally divided by a mov- 

 able septum (Fig. 77, s) in such a way, that the passage 

 on the right side of it leads to the carotid and systemic 

 arterial trunks, while the passage on the left side of it leads 

 to the third pair of trunks — namely, those ending in the 

 pulmonary and cutaneous arteries ; moreover, there is a 

 valve in the first of these two passages which tends to re- 

 tard the flow of blood (•»'). 



The consequences of these arrangements are as 

 follows : — 



When the auricles contract, the venous blood from the 

 right auricle {RA) is sent into both right and left pass- 

 ages of the bulb, but by the action of the valve (?'), and 

 by the stiucture of the carotid gland, the blood is checked 

 on the right side {ip), while on the left it runs freely into 

 the pulmo cutaneous trunks (rand/), and thus the respi- 

 ratory structures receive unmixed venous blood for purifi- 

 cation. 



As the lungs get gorged with blood, the resistance on 

 the two sides of the septum of the bulb becomes at first 

 equalised and soon becomes the greater on the left side ; 

 then the septum is forced over to the left, and the blood, 

 now mixed with pure blood, flowing in from the left 

 auricle, flows freely along the systemic arteries (2 and 2). 

 The obstruction of the carotid glands (t,i,'-/(^) being the 

 greatest and the last to be overcome, the carotid and 

 lingual arteries (t and /) receive the very last of the blood 

 — that, namely, which coming from the left auricle is 

 purely arterial— and in this way oxygenated blood only is 

 supplied to the head, sense organs, and brain. 



It should be borne in mind that in order to develop 



• " Beitrage zur vergleichenden Anatomic iind Physiologic der Gefiiss- 

 Systemes." In the third volume of the " Denkschriften der Mathematisch- 

 Natur-wissenchaftlichen dasse der Kaiscrlichen Akademie der Wissen- 

 schaftcn." Vienna : 1852. 



this most beautiful and complex apparatus, the co-or- 

 dinate development in due proportion of these beneficial 

 obstructions and checks must have been simultaneously 

 effected in order that their purpose should be duly served. 

 In other words, to account for its formation by an inde- 

 finite series of minute happy accidents would seem to 

 require such a successive occurrence of coincidences as to 

 become an improbability so great as to be indistinguish- 

 able from impossibility. 



St. George Mivart 

 {To be continued^ 



THE "CHALLENGER" EXPEDLTLON 



Bermuda 

 T7ROM the two visits made by the Challenger to 

 -'■ Bermuda we learn a good deal about the vegetation 

 of that island. Along the coast, which in some parts is 

 irregular and rocky, and in others of a sandy nature, 

 frequently with heaps of drifted sand, may be seen in 

 abundance a species of Borruhia, a low shrub belonging 

 to the compositeje, B. arborescens D.C. being common in 

 the West Indian Islands, and noted for having both 

 glabrous and silvery leaves on the same plant, as well as 

 the two forms on separate plants. In close proximity to 

 the Bon ichia was seen Tounicfortia giiaplialodcs R.Br., 

 a Boragineous shrub from 2 to 6 feet high, with white 

 flowers and downy leaves, and Ipomcva pes-eaprce Sw. 

 with its long stem, which frequently creeps to 100 feet or 

 more, and its purple flowers. In the crevices of the rocks 

 grow Euphorbia glabrata V., a shrubby glabrous plant 

 common to the West Indies, and on the shores of Florida, 

 Honduras, &c. A species of Tamarix is also abundant, as 

 well as ConocarpHs erectus L , and Coccoloba iivifcra Jacq., 

 known in the West Indies as the seaside grape, from the 

 violet-coloured, pulpy acid-flavoured peiranth ; an astrin- 

 gent extract like kino is likewise prepared from the bark, 

 and the bark itself is used for tanning leather. 



Many trailing plants scramble about on the sand dunes, 

 assisting to bind the loose sand together. Amongst the 

 most important of these is a hard, prickly grass, probably 

 a species of Cench us, Cakile ^r^'^ir//^ L.'Her, a singular 

 cruciferous plant allied to our Sea Rocket, and a species 

 of Sca-vola. The Mangrove \!\hizopIiora mangle L.) oc- 

 curs in swamps similar to those which have been so ol'ten 

 described by travellers ; but beside the true mangrove 

 swamps, there are others occupied by trees of Aviicni'' r, 

 A. nitida Jacq. being known in the West Indies as the 

 black, or olive mangrove. 



In the peat bogs, or marshes, which are surrounded by 

 low ranges of hdls, the most striking character of the 

 vegetation is the ferns ; species of Osmunda are abund- 

 ant, as well as Pteris aqiiilina L. .Some of the marshes, 

 however, have their special character of fern vegetation 

 some species, as for instance Acrostichiiin aureuin L. [Cliry- 

 sodium vulgare Fee), being found only in particular spots. 

 The Junipers {Junipcrns hcrnuidiana Lun.) also thrive 

 in the marshes, but none of the trees at present standing 

 approach in size those that are occasionally found below 

 the surface. These large trunks usually lie at a depth of 

 about two feet. The average diameter of the trunks of 

 existing trees may be taken at from two to three feet, and 

 these are mostly unsound in the centre owing to the 

 marshy ground in which they grow. The largest known 

 living trees in the island measure respectively fifty-nine 

 inches and thirty-nine inches in diameter ; the first is 

 hollow, but the second is apparently sound. Amongst 

 other noticeable marsh plants are Myrica cerifera L., a 

 shrub the berries of which, in Central America, yield 

 wax from which candles are made, and Rliits toxico- 

 dendron L., the Poison Oak of North America. 



In the fresh-water ponds or lakes inland, some of which 

 are a quarter-mile long, and often are in close contiguity 



