70 



STUDIES IN COMPARATIVE ANATOMY. 



The two superior venae cavae in the elephant, as in monotremata, 

 marsupials, many rodents, the hedgehog, and the bat, and by a rare 

 deviation from the ordinary rule in the human subject also, are 

 explicable as a retention of an embryonic structure very general in 

 vertebrates. At first the blood is returned to the heart by an anterior 

 (jugular) and a posterior (cardinal) pair of venous trunks, as weU as 

 by a median posterior, wliich persists and constitutes the inferior cava. 

 The cardinal and jugular veins of each side unite to form a precaval 

 or canal of Cuvier, and the two precavals are primitively united into a 

 common trunk wliich enters the undivided auricle. The common 

 precaval trunk ultimately forms part of the right auricle into which 

 the two precavals then open separately, forming the right and left 

 superior (or anterior) venae cavae. A transverse connection is next 

 established between the jugular veins; the left jugular, below the 

 transverse branch, and the left precaval contract and become obUter- 

 ated ; while the enlarged transverse branch becomes the left innomi- 

 nate vein, and the lower part of the right jugular together with the 

 right precaval forms the single superior (or anterior) vena cava of the 

 higher mammalia. The coronary sinus is, according to Mr Marshall, 

 the only part of the left anterior cava which in these mammals re- 

 mains pervious. (See Kathke, Meckel's Archw. 1830, p. 63; and 

 Ueber den Bau unci die Entwickehmg des Venensystems der 

 Wirhelthiere, Konigsberg, 1838; Bardeleben, Miiller's Archiv. 1848; 

 Marshall, Phil. Trans. 1850, part i. ; and Owen, Anatomy of Verte- 

 brates, vol. iii. p. 551.) 



Right Ventricle. — The " incomplete septum " of Vulpian and 

 Philipeaux seems to consist merely of the more or less united 

 fleshy columns. In our example these differ considerably from 

 the description of the authors just named. Four or five are 

 attached, low down, to the external wall ; three or four others, 

 less distinct, from the ventricular wall to the interventricular 

 septum. All the very numerous tendinous cords spring from 

 fleshy columns, though these have not on the inner side of the 

 ventricle any considerable free extent. No corpora Arantii are 

 present on the semilunar valves of the pulmonary artery. The 

 wall of the ventricle is cavernous towards the tricuspid valve, 

 and also along a line leading from the apex to the orifice of the 

 pulmonary artery, close to the ventral border of the interven- 

 tricular septum. The tricuspid valve may have one or two (in 

 our example two) small additional cusps. 



Left Auricle. — The entry of the pulmonary veins seems to vary. 

 Vulpian and Philipeaux found two openings into the auricle — 

 a small internal and a large external, which latter received 

 three of the pulmonary veins. Dr Watson describes four 



