150 BRAIN AND BODY OF FISH 



included in this type, pain being wholly protopathic. Some 

 physiologists state that there are definite nerve-endings for pain, 

 while others regard pain as a quality which may be present in any 

 sense and not as itself a true sensation. 



The free nerve-endings among the cells of the epidermis are 

 probably the pain receptors, because these endings alone are present 

 in some parts of the body where susceptibility to pain is the only 

 sense quality usually present, such as the dentine and pulp of the 

 teeth and the cornea of the eye. A section of the tooth of a fish 

 shows the outer layer of dentine and fine nerve terminals entering 

 the cavity, and ending free. 



Again in a section of the cornea there are various plexuses in the 

 connective tissue and sub-epithelial layers which end in an intra- 

 epithehal plexus terminating in naked fibrils, often varicose, amongst 

 the superficial cells. Similar endings are found tliroughout the 

 epidermis. Skin-pain is, therefore, probably received by free 

 nerve endings lying between the superficial cells and is wholly of 

 the protopathic type. But it must be borne in mind that excessive 

 stimulation of other sense organs may give rise to pain, although in 

 such cases the pain is accompanied by the usual sensation of the 

 particular organ. For example, a very loud noise may cause acute 

 pain, but it retains, nevertheless, the character of sound. 



The epidermis of fish is, of course, characterised by its scales, 

 which is in sharp contrast to the appendages of the human skin, 

 hairs and their sebaceous glands, and the sweat glands. Before 

 mentioning the special types of sense organs peculiar to the epidermis 

 of fish we must refer to the end-organs of chemical sensibihty found 

 in man only on moist epithelial surfaces, but in fishes they may be 

 present over the entire surface of the body. But the most character- 

 istic of the sensory organs of fish are taste-buds and the organs of 

 the lateral line. The former are composed of cells arranged like 

 the segments of an orange which are enclosed in a sheath which 

 leads to the surface by a pore so as to allow access to the tips of 

 the sensory cells. They are most in number on the li23s and barbels, 

 but are found all over the body and on the fins. 



These important sense organs are also associated with tactile 

 sensation, as such fishes as the barbel, gudgeon, and loach have 

 prominent barbels, which they use in hunting for food in gravel 

 and stony bottoms, and it is noteworthy that in these fish the nerves 

 divide within the brain, so as to lead to special areas of the facial 

 lobe, suggesting a differentiation of function. Pit-organs is the 

 term appHed to a type of sensory organ found either isolated or in 

 a long connected linear tube, called the lateral line, often clearly 



