PLACENTAL STRUCTURES OF THE TENREC. 307 
ment, now ordinarily' made, to the effect that the Cetacea have no azygos veins at all, 
as this peculiarity would, if it really did exist, differentiate them from all other mam- 
mals whatever. But on looking at the words of Von Baer’, upon whose authority this 
statement is made, and at the facts with which Von Baer’s words are usually in accor- 
dance, it will be found that the vena-azygos system not only does exist in Cetacea, but 
actually furnishes us with an additional point of affinity between them and the large 
ungulate Mammalia. Von Baer’s words, in the ‘ Bulletin de Académie Impériale des 
Sciences de St. Pétersbourg,’ l. c., are, ‘‘la veine dite impaire qui manque eacepté le bout 
antérieur, et dont les sinus de la colonne vertébrale tiennent lieu.” ‘This anterior end, 
however, is a very considerable vessel ; and in the ‘ Nova Acta’ (J. c.) I find that Von Baer 
describes it as having a trunk as large as that of the vena cava superior, which it joins ; 
and he goes on to state that its large size is not to be accounted for by its intercostal 
factors, but by the junction to it of a trunk from within the vertebral canal, of large size 
and connected by its constituent factors, as the vena azygos is, with those of the lower 
cava as also with the lower intercostal veins. If we examine a Pig, we shall find it to 
possess a large azygos vein on the left side ; and this the Cetacean, it is true, does not ; 
but it will be seen to have on the right side a short vena azygos, just as the Porpoise 
has, with four or five affluents from the upper intercostal spaces, as well as very con- 
siderable tributaries from the muscles of the back and scapule. The vena azygos 
system is well known to be intimately connected with venous ramifications situated 
about, around, between, and within the spinal column ; and the greater development of 
any one of these vascular connexions does not destroy the specific and distinctive 
character of that peculiar system. Its perirrhachidian or dorsispinal anastomosis is 
chiefly developed in the right side in the Pig, which has a left azygos also; its endo- 
rhachidian or intravertebral factors have absorbed its lower intercostal tributaries in the 
Porpoise ; and in this animal there is no vena azygos on the left. These points of 
difference are, however, but slight as compared with the difference which the statements 
ordinarily made on this point would lead us to expect. The Cetacea, we may add, have 
been observed to resemble the Sheep and Pig and Horse, in the deficiency of the rudi- 
mentary structure known as the Eustachian valve, which, however, is by no means 
invariably present in deciduate Mammals. 
A few points of resemblance between the placentee of the Ungulata and the Mutica 
have escaped notice. First, in Cetacea and in certain Ungulata we find the mem- 
branes of what is often a solitary embryo prolonged from one cornu round into the 
other, and projecting by a cecal extremity into the short corpus uteri. Such a condi- 
tion of the structures is figured from the Mare by Colin (J. c.), and has been seen by 
myself in the membranes of a small Cetacean, sp. ? ; and in the Cow and other Ruminants 
1 «Phil, Trans.’ for 1849, p. 152. Milne-Edwards, ‘ Physiologie Comp.’ vol. iii. pl. 2. p. 598. 
? Nova Acta, vol. xvii. pt. 1. p. 408, 1834; Bull. Acad. St. Pétersbourg, tom. i. 1835; Froriep, ‘ Notizen,’ 
50, p. 38. 1836. 
VOL. V.—PART IV. 2s 
