38 POULTRY DISEASES 



The sympathetic nerves have their origin at tlie cranio- 

 cervical ganglion which lies at the base of the skull. From 

 this ganglion fine filaments are sent to most of the cranial 

 nerves as well as connecting branches. The sympathetic trunk- 

 extends along the cervical vertebra, oecup3'ing an ol)liqiie 

 canal. It exchanges filaments with the cervical nerves. Jt 

 continues on either side of the base of the vertebra through 

 the thoracic and abdominal cavities. Branches from these 

 trunks go to form the splanchnic nerve system supplying the 

 visceral organs of the abdominal cavity. The right and left 

 trunks finally unite, forming the ganglion coccygeum. 



ORGANS OF SPECIAL SENSE 

 The Eyes 



The eyes are rather large in proportion to the size of the 

 ])ody. The sense of sight is well developed. The eyes stand to 

 the side of the head in the domestic birds. The orbital cavities 

 are separated laterally by a bony septum (the septum inter- 

 orbitale), but are not completely surrounded by ])one. The 

 lower eyelid is the larger and often incloses a small cartilagi- 

 nous plate. The lower lid is more freely movable than the up- 

 per. The third eyelid (membrana nictitans) is well developed. 

 A special muscle draws it from the inner canthus of the 

 eye over the ball. There are no tarsal glands as in mammals. 

 The muscles of the eyelids are of the smooth variety and in 

 the place of eyelashes there may be found minute feathers. 

 The gland of the eyelid lies on the nasal side and is often 

 larger than the lacrimal (tear) gland. Its secretion is poured 

 out onto the third eyelid. The tear gland (lacrimal) lies in 

 the angle close to the temple. The tear gland is small and 

 has one or more ducts emptying into the conjunctival sac at 

 the temple side of the eyeball. The lacrimal sac and lacrimal 

 duct carrying the tears to the nasal cavity are similar as those 

 of mammals. The eyeball is made up of three coverings. The 

 cornea covers the anterior portion of the eye and the sclera 

 the outer remaining portion of the eyeball. The sclera is made 

 up of small overlapping scales, connective tissue and a car- 

 tilaginous cup extending from the o]^tic nerve at the posterior 

 pole to the equator of the globe. The scleral ring is at the 

 anterior border of this cartilage. The anterior chamber of 

 the eye is large. The middle layer of the posterior portion 

 of the eyeball is called the choroid, and is rich in pigment 

 and blood vessels. Tlie retina forms the inner coat and is rich 

 in a dark pigment. The ciliary ])ody consists of many folds. 

 Tlie ciliary muscle consists of oblique fibers arranged in three 



