CHAPTER XXI. 



DEVELOPMENT OF 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 Eoof. — 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 tho 



