CHAPTER XXI. 



DEVELOPMENT OF THE SENSE-ORGANS. 



Origin of the most highly Purposive Sense-organs by no Preoonoeivo<3 

 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 Roof. — 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 thf 

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

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

 brane. — Origin of these from the First GiU-opening and the Parte 

 immediately round it (the First and Second Gill-aroh). — Eudimentai ; 

 Outer Ear. — Eudimentary Muscles of the Ear-sheU. 



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

 ment, and unless this is more complete, can never make r%pid progress j for 

 the history of development furnishes the philosopher with the materials 

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

 anatoniical and physiological researches should be prosecuted more from th« 



