158 MORPHOLOGICAL TYPES 



formed from the 4th and 5th cervical spinal nerves, and run 

 back to the diaphragm on each side of the heart. The length 

 of their course shows the amount which the diaphragm, 

 together with the heart and aortic arches, have moved back- 

 wards during development ; a movement which has already 

 been noticed in connexion with the recurrent laryngeal nerves. 



Sense-organs. — The sense-organs of the mammal show 

 certain peculiarities. The sensory surface of the olfactory 

 organs is increased by the formation of folds supported by the 

 turbinal bones. Jacobson's organ opens into the mouth in 

 some forms, but it disappears in others. The ear is remarkable 

 for the external pinna, and the inclusion of the articular and 

 quadrate as the malleus and incus, in the chain of bones which 

 together with the stapes (columella auris) connect the tympanic 

 membrane with the fenestra ovalis. The projection from the 

 saccule which forms the ductus cochlearis in lower forms, and 

 is responsible for hearing as apart from appreciating balance 

 (the function of the rest of the ear), is in the mammals very 

 highly developed. It is much elongated, and is coiled in a 

 spiral which enables it to be accommodated in the compara- 

 tively small cochlear part of the auditory capsule. 



The eyelids are movable and muscular, and well supplied 

 with glands ; lachrymal and Harderian glands are present, 

 and a naso-lachrymal duct. 



Characteristics of Lepus, typical of Mammals : 



Hair ; 



Bones with diaphysis and epiphyses ; 



Two condyles to the skull ; 



Loss of coracoid ; 



Tympanic bulla ; 



Lower jaw composed of dentary only ; 



Teeth heterodont and diphyodont ; 



Articulation of dentary with squamosal ; 



Conversion of articular and quadrate into malleus and 



incus ; 

 Diaphragm ; 

 Single left aortic arch ; 



