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A MANUAL OF VETERINARY PHYSIOLOGY 



ciliary muscle consists of both circular and longitudinal (or 

 meridional) fibres ; in the horse, and probably all the lower 

 animals, only meridional fibres exist. The muscle is composed 

 of unstriped fibres, is innervated by the third pair of nerves, and 

 its use is to pull the choroid forward. The object of this will be 

 apparent when the question of accommodation is discussed. 



The Vitreous humour is enclosed in the hyaloid membrane. 

 Anteriorly this membrane, here known as the zonule of Zinn, 



becomes dovetailed into the 



Rods. 



Cones. 



ridges formed by the ciliary 

 processes, and, enveloping 

 the lens, forms its suspen- 

 sory ligament. If the amount 

 of vitreous humour present 

 is sufficient in quantity, 

 this ligament of the lens 

 must always be tense, and 

 as it is very inelastic, it 

 tends to keep the lens 

 flattened. The matter will 

 be referred to again in 

 speaking of accommodation. 

 The Retina lies within the 

 choroid and outside the 

 vitreous humour ; it spreads 

 out from the entrance of 

 the optic nerve of which it 

 is the expansion. Micro- 

 scopic examination shows 

 this membrane to be com- 

 posed of seven layers (Fig. 

 1 60), of which the most 

 important is one termed 

 from its appearance the 

 layer of rods and cones. 

 It has been shown con- 

 clusively that these rods 

 and cones are the essential elements of the retina, and that 

 wherever they are absent the part is insensitive to light, as, for 

 example, at the entrance of the optic nerve which forms the 

 blind spot. Though the layer of rods and cones is the most 

 important, it is not placed, as one would suppose, next the vitreous 

 humour, but next to the choroid, whilst the layer next to the 

 vitreous humour is composed of nerve fibres and ganglion cells. 

 Rays of light have, therefore, in the first place to pierce the 

 entire thickness of the retina in order to arrive at the rods and 



Fig. 160. — Diagram of Structure of 

 Retina (Bowditch, after Cajal). 



A , Layer of rods and cones ; B, external 

 nuclear layer ; C, external molecular 

 layer ; E, internal nuclear layer ; F, in- 

 ternal molecular layer ; G, layer of gang- 

 lion cells ; H, layer of nerve fibres. 



