114 ESSENTIALS OF PHYSIOLOGY. 



substance. A tube is inserted into the cylinder for varying distances, 

 so that a greater or less part of the cylinder is exposed to the air which 

 passes through the tube. The end of the tube outside the cylinder is 

 placed in a nostril, and the smallest amount of exposed cylinder surface 

 which will give a sensation indicates the threshold stimulus for the 

 substance tested. 



The olfactory sense is developed to a varying extent in different 

 animals. Generally speaking, it is present to a greater degree in many 

 of the lower animals than in man ; in the dog, for example, it is 

 highly developed. 



SECTION III. 

 THE SENSE OF SIGHT. 



The end-organs of the sense of sight are situated in the eyeball, 

 which is protected from injury by its situation in the orbital cavity and 

 also by the eyelids. The surface of the eyeball is kept moist by the 

 tears, which are secreted by the lachrymal gland. Loss of moisture 

 occurs through evaporation from the surface of the eyeball, and super- 

 fluous tears are drained away through the punctci lachrymalia at the 

 inner end of the eyelids into the lachrymal sac, and thence by the nasal 

 duct into the nasal cavity. The lachrymal secretion is slightly alkaline 

 and contains sodium chloride. 



THE EYEBALL. 



The eyeball consists of three coats surrounding the transparent 

 media which constitute the dioptric apparatus. It is covered in front 

 by the conjunctiva, a connective-tissue membrane with a superficial 

 layer of stratified squamous epithelium. The conjunctiva is reflected 

 over the posterior surfaces of the eyelids, and is represented on the 

 cornea only by the layer of stratified epithelium. The outer coat of 

 the eyeball consists of the solera and cornea. The sclera forms five- 

 sixths of the coat, and is protective in function. It is opaque, and is 

 made up of dense fibrous tissue with some elastic fibres and flattened 

 cells, some of which are pigmented. The cornea forms one-sixth of the 

 outer coat, and has a somewhat greater convexity than the sclera. It 

 is transparent, and is made up of parallel lamellae of white fibrous 

 tissue with spaces between, in which lie flattened branched cells, the 

 corneal corpuscles. It is covered in front by stratified squamous 

 epithelium, which rests on a homogeneous-looking membrane composed 

 of closely woven fibrils, and known as the anterior elastic lamina. The 

 posterior surface of the cornea is covered by a single layer of flattened 



