568 COMPARATIVE ANATOMY 



are sometimes solitary, sometimes clustered. On the other hand, the 

 Grandry's corpuscles of birds are encapsuled, and the nerve termination 

 lies between two tactile cells. 



In the relatively small corpuscles of Krause and in the large one of 

 Pacini, both found in mammals, the nerve termination is club-shaped 

 and encapsuled. In a Pacini corpuscle, are as many as eight concentri- 

 cally arranged lamellae. A secondary or adjunct nerve fiber penetrates 

 the capsule and forms a varicose network within the inner lamella. 

 Pacini's corpuscles are located not only in the deeper layers of the skin, 

 but also in the mesenteries, tendons, and periosteum. Those present in 

 tendons give rise to sensations which serve to indicate the position of a 

 limb. Those in the deeper parts of the body probably give rise to painful 

 sensations associated with disease. 



The Golgi-Mazzoni corpuscles are spherical or club-shaped tactile 

 corpuscles, in which the coiled nerve termination is enclosed by, but not 

 in contact with, the surrounding capsule. In them, both chief and 

 adjunct nerve fibers are present. These corpuscles occur in the corium, 

 the peritoneum, and in the conjunctiva of the eye. The genital cor- 

 puscles found in the corium of the glans penis are supposed to be the 

 sensory mechanism associated with sexual desire. 



Development of Cutaneous Sense Organs 



Free nerve terminations are formed by the outgrowth of sensory 

 nerve fibers from the sensory ganglia. Of these, some remain free, some 

 effect connexion with tactile cells, some become enveloped by connective 

 tissue capsules. 



LATERAL LINE ORGANS 



Lateral line organs are a specialized type of cutaneous sense organs 

 limited to fishes and water-dwelling amphibia. It is believed that they 

 respond to currents of water and to sudden changes in pressure. Although, 

 among cutaneous sense organs, the arrangement in rows is peculiar 

 to the lateral line organs, this linear arrangement is presumably second- 

 ary, since both invertebrates and vertebrates have, scattered over the 

 surface of the body, sensory papillae or neuromasts similar to those 

 of the lateral line organs. 



Dorsal, lateral, and ventral rows of lateral line organs occur in verte- 

 brates on each side of the body. Usually all three are present only in 

 embryonic and larval stages. The lateral rows persist in the trunk 

 region of adult fishes and urodeles. Three lines, however, are present 

 in the head, supraorbital, infraorbital, and mandibular, innervated by 

 branches of the facial and vagus nerves, and exceptionally by a branch 



