29 



great proportional size, especially in longitudinal extent, but the proper cranial 

 cavity is small : it does not extend forwards beyond the posterior third part of 

 the skull, and is separated from the upper, lateral, and even from the posterior 

 and under surfaces of the skull by large air-cells. To use a botanical compari- 

 son, one might say that the cerebral pith or kernel, besides having its immediate 

 hard shell in the vitreous table, is also defended by a thick cellular husk and 

 a tough outer capsule. 



The prodigious extent of the air-cells continued from the frontal and ethmoidal 

 sinuses, form, indeed, one of the most striking characters in the cranial organi- 

 zation of the Mylodon : they are continued backwards into the parietal, tem- 

 poral, sphenoid and occipital bones, separating the two tables of the skull for 

 an extent of sometimes two inches ; irregularly sinuous bony plates form the 

 medium of support and connection of these tables, divide the air-chambers, and 

 multiply the surface of the vascular lining membrane. 



The cerebellar division of the cranial cavity may be traced by a slight projec- 

 tion of the inner wall at its circumference : but this is not developed into a bony 

 tentorium. It is of large proportional size, and shows that the cerebellum was 

 wholly posterior to the cerebrum. The small proportion of the carotid to the 

 basilar artery, manifested by their respective bony canals, also indicates the low 

 development of the higher or cerebral division of the encephalon. The sella 

 turcica is wide and very shallow, undefended by clinoid processes. The cribri- 

 form plate is of great extent, and divided by a crista galli : the anterior com- 

 partment of the cranial cavity in which this plate is lodged, indicates the large 

 size of the olfactory lobes. 



The two fractures before described are limited to the outer table of the skull, 

 and have laid open only the air-sinuses. Of the inflammation and disturbance 

 thence ensuing, abundant evidence is afforded by the vertical section of the 

 skull through the anterior healed fracture, represented in Plate V. fig. 2./. The 

 outer table and contiguous septa of the air-sinuses have been thickened ; the 

 sinuses have been distended with collections of pus, their walls have yielded, 

 modified by the absorbent and restorative processes, and the vomer {d) has been 

 bent considerably to the left side. It resumes its ordinary median vertical posi- 

 tion anterior to this seat of injury, and extends to near the anterior extremity of 

 the skuli, forming there a stout supporting wall to the large nasal bones. 



