78 MAMMALIAN DESCENT. [Lect. III. 



in Man is the greatest, as it is one of the principal 

 means of supply of fresh blood to the braia. 



Anyhow, as the brain widens and grows, in the ascent 

 of the types, these arteries get further from the mid- 

 line at this their entrance into the skull. In Birds and 

 Eeptiles the pituitary body drops into a hole, — does not 

 lie on a saddle as in us, — and in them the internal 

 carotids run up through the open space, close beneath 

 the pituitary bag. 



Thus, we see that the Marsupials are intermediate 

 between the Sauropsida and the Eutheria ; they have an 

 imperfect seat to their "sella turcica" (turkish saddle, 

 as it is generally called in Man), and the iaternal 

 carotids pass inwards, as in the oviparous types, but 

 rather further apart. 



Cliaracter 6. — The peculiar form of the malleus 

 (hammer) seen in many Marsupials. Its large size, 

 and the sickle-shaped tympanic fork of the processus 

 gracilis, is seen in low Eutheria, e.g., the Mole, for a 

 time. 



The growth of periosteal or superficial bone is an 

 attempt to form the usual splints of an ordinary 

 mandible in the ovipara. Sometimes the three plates — 

 external articular, angular, and supra-angular — are quite 

 distinct. . These are formed just as the cartilaginous rod 

 is being pinched in or starved ofi", so that the fore part 

 of the jaw becomes segmented from the hind part, does 

 all the mandibular functions, and leaves the hinder, or 



