ARTERIES OF MAMMALIA. .047 



plexuses upon the humerus, mainly expended in nourishing the 

 bones, their ligaments, and the enveloping integument. l The caudal 

 prolongation of the aorta is surrounded by layers of plexiform arte- 

 rioles, as by a sheath, in its course along the wide haemal canal. 2 



The littoral and herbivorous Sirenia, which never go so deep or 

 stay so long submerged as the Whales, seem not to possess the 

 intercostal and myelonal arterial plexuses : at least I found them 

 not in the Dugong. 3 



Amongst the minor degrees of plexiform multiplication of 

 arterial canals is that oldest recorded instance 4 of the intracranial 

 6 rete mirabile ' at the base of the skull in Ruminants : it is large 

 in grazers, is less in browsers, and least in the Giraffe which 

 habitually feeds with the head raised. In Bovidce, where the 

 ' rete' is most extensive, it is situated at the sides and back of the 

 sella turcica, in the sclerous venous receptacle called ' cavernous 

 sinus.' There is no definite bifurcation of the cephalic arterial 

 trunk into an ecto- and ento-carotid, in Ruminants: a small 

 branch of the carotid, perforating the cranium at, or behind, the 

 foramen ovale, represents the ( middle meningeal artery,' and joins 

 the 'rete mirabile'; but this is mainly formed, in advance, by 

 branches of the internal maxillary, which enter the fissura lacera 

 anterior, subdivide and anastomose reticularly, and are continued 

 backward, on each side of the ' sella,' as a ' rete mirabile ': three 

 or four transverse portions bring the main lateral ( retia ' into union 

 with each other ; while, posteriorly, are sent off a median and 

 two lateral narrow plexiform extensions : the latter diverge to 

 anastomose with the precondyloid arteries ; then the middle por- 

 tion is continued to join the converging cerebral branches of the 

 vertebral arteries. These, in the Ox, enter the neural canal 

 between the axis and third cervical, are united together by oblique 

 cross-branches, as they advance ; each bifurcates in the neural 

 canal of the atlas, sending one branch out through the anterior 

 perforation of the neurapophysis, while the other converges towards 

 its fellow, to terminate by anastomosing with the median pro- 

 duction of the ' rete ' from the ' circle of Willis,' and also with a 

 division of each precondyloid artery. 



In the Hog (Sus scrofa), a larger proportion of the com- 

 paratively small ' rete mirabile' is formed by the vessel entering 

 the cranium through the posterior ' fissura lacera,' representing 

 an e entocarotid.' A common efferent vessel, piercing the inner 



1 xciv. p. 366 (note). 2 cxcvm". 



3 cxvn". p. 35. The trace observed by Stannius in the Manatee does not support 

 a functional inference, as in the true whales. * cc". 



