REPTILIA 573 



sacs embedded in the musculature of the ventral side of the base 

 of the tail. They are elongated, smooth-walled structures which 

 open into the posterior chamber of the cloaca. During copulation 

 these organs protrude through the vent of the cloaca by everting 

 themselves; i.e., turning themselves wrong side out. They are pulled 

 back into place by the contraction of a retractor muscle attached to 

 the distal end of each organ. Often only one hemipenis functions 

 during copulation, but this is not an invariable rule. 



The female reproductive organs consist of a pair of ovaries and a 

 pair of oviducts. Each ovary is attached to the dorsal body wall in 

 the posterior end of the body cavity by means of a double sheet of 

 peritoneum, the niesovarium. The folded oviducts lie dorsolateral 

 to the ovaries. The anterior end of each oviduct is expanded into a 

 thin-walled ostium; posteriorly, each duct opens into the dorsal wall 

 of the posterior cloacal chamber. The oviducts are held in position 

 by a broad fold of the peritoneum, the hroad ligament. 



The common cloacal chamber of both sexes receives the products 

 discharged by the alimentary canal, the kidneys, and gonads. The 

 cloaca is divided into two regions, an anterior and a posterior, by 

 an incomplete ridge of mucous membrane. 



The Nervous System 



The reptilian nervous system shows certain advances over the 

 nervous system of amphibians. Twelve pairs of cranial nerves are 

 present, as in birds and mammals. The various regions of the 

 brain are better developed than in lower vertebrates ; however, the 

 reptilian brain is still a comparatively simple structure. 



The nervous system of horned lizards, as in other vertebrates, 

 consists of the central nervous system, the peripheral nervous sys- 

 tem, and the sympathetic nervous system. 



The central nervous system consists of a 'brain located within the 

 skull, and a spinal cord located within the neural canal. The brain 

 has all the divisions of the typical vertebrate brain. The peripheral 

 nervous system is composed of the twelve pairs of cranial nerves from 

 the brain and sixteen pairs of spinal nerves from the spinal cord. 

 These nerves are arranged segmentally. Typically, there is one pair 

 to each segment. The sympathetic system includes two conspicuous 

 white trunks that extend along each side of the vertebral column 

 and the lateral fibers from these trunks. Each of the sympathetic 



