52 ABNORMAL CONDITIONS OF THE AXIS. 



bosces are either very rudimentary or wanting. When the ears 

 only are present, the squamosal bones form a shield in front of the 

 skull-case. The term nanocephdlus has been applied to this form. 



The periotic bones always retain their normal position with 

 regard to the axis, and the tympanic bones are usually removed 

 from them. The pharynx always ends blindly above, but often 

 communicates with the exterior by tympanic spiracles. 



The brain is either arrested in its development or, in nano- 

 cephalus, is completely absent, the skull-cavity being occupied 

 by a cyst surrounded by the anterior termination of the medulla. 

 In cyclocephalus in the human subject the axis of the skull re- 

 mains in the same line as that of the vertebral column. 



The most extreme form of cranial arrest produces entire ab- 

 sence of the skull, or a cyst between the external ears terminates 

 the axial canal in front. The term acephalus has been used to 

 designate this malformation. 



a. Cyclocephalus. 



200. The head of a male foetal Pig with a frontal proboscis and 



a pair of eyes fused as far as the cornea. The maxillae are 

 represented by a papilla beneath the eyes. The mandible 

 is absent. The tympanic cavities are united. The ex- 

 ternal auditory meatuses are not fused together : a black 

 bristle is passed through that on the left side into the 

 pharynx. The pharynx has been laid open. The animal 

 was otherwise normal. 



201. A similar preparation in which the eyes are fused into a 



single globe. The ear, pharynx, and larynx are dissected. 



The hyoid arch is perfect. 



On the right side the anterior wall of the external meatus only 

 has been removed, on the left side the tympanum has also been 

 laid open. There was an external opening beneath the bullae into 

 two distinct passages which lead into the pharynx ; these auditory 

 spiracles are marked by two bristles. The median Eustachian tube 

 is also marked by a bristle. The tympanic membranes were both 

 perfect. 



202. The head of a foetal Sow with similar deficiency. In this 



specimen the median eye appears as if it had been much 

 damaged by inflammatory action. 



203. A section through the head of a similar foetal Pig, showing 



