CHAPTER XXI 



DEVELOPMENT OP THE SENSE-ORGANS. 



Origin of the most highly Purposive Sense-organs by no Preconceived 

 Purpose, but simply by Natural Selection. The Six Sense-organs and 

 the Seven Sense-functions. All the Sense-organs originally Developed 

 from the Outer Skin-covering (from the Skin-sensory Layer). Organs 

 of the Pressure Sense, the Heat Sense, the Sexual Sense, and the 

 Taste Sense. Structure of the Organ of Scent. The Blind Nose-pits 

 of Fishes. The Nasal Furrows change into Nasal Canals. Separation 

 of the Cavities of the Nose and Mouth by the Palate Hoof. Structure 

 of the Eye. The Primary Eye Vesicles (Stalked Protuberances from 

 the Twixt-brain). Inversion of this Eye Vesicle by the Crystalline 

 Lens, separated from the Horn-plate. Inversion of the Vitreous Body. 

 The Vascular Capsule and the Fibrous Capsule of the Eyeball. Eye- 

 lids. Structure of the Ear. The Apparatus for Perception of Sound : 

 Labyrinth and Auditory Nerve. Origin of the Labyrinth from the 

 Primitive Ear Vesicles (by Separation from the Horn-plate). Conduct- 

 ing Apparatus of Sound : Drum Cavity, Ear Bonelets, and Drum Mem- 

 brane. Origin of these from the First Gill-opening and the Parts 

 immediately round it (the First and Second Gill-arch). Rudimentary 

 Outer Ear. Rudimentary Muscles of the Ear-shell. 



" Systematic Physiology is based especially upon the history of develop- 

 ment, and unless this is more complete, can never make rapid progress ; for 

 the history of development furnishes the philosopher with the materials 

 necessary for the secure construction of a system of organic life. Hence 

 anatomical and physiological researches should be prosecuted more from the 



