98 AN INTRODUCTION TO ZOOLOGY 



The second is a pair of muscles arising together from the inner an- 

 terior corner of the orbit. One, the Obliquus superior, runs obliquely 

 upwards and backwards, and the other, the Obliquus inferior, runs 

 obliquely downwards and backwards to be inserted on the eyeball. 

 Between them these six muscles, which are to be found in all verte- 

 brate animals, can move the eye in any direction that the animal 

 wishes. The nerve supply of these muscles, which is also constant 

 in vertebrates, has already been indicated.* In the frog there is an 

 additional muscle present, called the Retractor bulbi, which surrounds 

 the optic nerve and enables the whole eyeball to be drawn back 

 somewhat into the orbit. 



The senses of touch and taste are served by cells which, 

 unlike those of the other three senses, are not aggregated together 

 to form fairly large well-defined sense organs, but are scattered singly 

 or in small groups over a more or less wide area. Touch is located 

 partly in the epidermis, in which nerve fibres form a fine network 

 with ends situated between the cells, and for the most part in the 

 dermis. The tactile and organs of the dermis comprise corpuscles 

 of various sorts, but each consists essentially of a much modified 

 cell or cells, in close relation to which a nerve fibre breaks up into a 

 fine mesh work of arborisations. By means of these different organs 

 the various stimuli that constitute " touch," i.e. pressure, warmth, 

 cold and pain, are received and conveyed to the brain. It will be 

 noted that the stimulus only reaches them in a reduced form through 

 the epidermis, with the result that when an accident removes a portion 

 of our epidermis all the stimuli, including pain, appear much mag- 

 nified when anything is touched by the injured spot. The gustatory 

 end organs subserving taste are naturally limited to the tongue 

 and mucous membrane of the mouth and pharynx. Generally 

 they consist of a group of modified columnar cells, some of which 

 give off branching processes that are related to the ends of the 

 nerves of the plexus underlying the epidermis. The sense of taste 

 is resolvable into at any rate four primary components : Sweetness, 

 Bitterness, Acidity and Salinity. 



The auditory organ or organ of hearing in the frog consists 

 of two separate parts, known respectively as the inner ear and the 

 middle ear ; the former is the essential organ of hearing, while the 

 latter is concerned with receiving sound waves and conveying 

 them to the inner ear. The inner ear is constituted by a somewhat 

 complex structure known as the membranous labyrinth, and this 

 is lodged in the otic capsule. The labyrinth consists of a membranous 

 sac, the vestibule, divided into two parts. The upper larger portion 



* The muscles of the vertebrate eye can be studied more readily in 

 Scy Ilium, on account of its size. 



