SENSE OR RECEPTOR ORGANS 841 



The general arrangement of these ganglia and the autonomic nerve fibers 

 to the spinal nerve series is shown in figure 358A. It is to be observed that only 

 two neurons, a preganglionic and a postganglionic, are involved in the efferent 

 chain regardless of the number of ganglia traversed. 



E. The Sense or Receptor Organs 



1. Definition 



The sense organs are the sentinels of the nervous system. Endowed par- 

 ticularly with that property of living matter known as irritability, they are 

 able to detect changes in the environment and to transmit the stimulus thus 

 aroused to afferent nerve fibers. However, the perceptive ability of all sense 

 organs is not the same, for specific types of sense receptors are developed spe- 

 cialized in the detection of particular environmental changes. 



There are two general areas of sensory reception, viz.: (1) The somatic 

 sensory area, and (2) the visceral sensory area. The location of somatic and 

 visceral areas in the myelencephalon are shown in figure 3551. 



The somatic sensory organs are associated with the general cutaneous sur- 

 face of the body and also in tissues within the body wall. Consequently, this 

 area may be divided for convenience into two general fields, namely, ( 1 ) 



Fig. 359 — Continued 



terminal ends of one or more unmyelinated fibers, and also, in addition, the terminal 

 end of a myelinated fiber which loses its myelin as it enters the axial core of the corpuscle. 

 Side branches arise from the central core of nerve fibers. Found in deeper parts of dermis, 

 and also in association with tendons, joints, intermuscular areas as well as in the 

 mesenteries of the peritoneal cavity, and the Hnings of the pleural and pericardial 

 cavities. (D) Nerve endings in skin and hair follicles. As the myelinated fibers enter 

 the skin they break up into smaller myelinated fibers. After many divisions the myelin 

 sheaths are lost, and finally the neurilemma also disappears. The free nerve endings 

 enter the epidermis and after other divisions form a network of terminal fibers among 

 the epidermal cells. Below the stratum germinativum of the skin, some of the fibers 

 terminate in small, leaf-like enlargements around the hair-follicles below the level of 

 the sebaceous glands. (A-D, redrawn and somewhat modified from Ranson, 1939, The 

 Anatomy of the Nervous System, Philadelphia, Saunders.) (E) Part of longitudinal 

 section of the lateral line canal of a Mustelus "pup" at the level of the first dorsal fin. 

 Observe termination of nerve fibers among groups of sensory hair cells. The lateral line 

 canal communicates with the surface at intervals by means of small tubules. (Redrawn 

 and modified from Johnson, 1917, J. Comp. Neurol., 28.) (F) Transverse section of 

 lateral line canal, higher rrtagnification, showing termination of nerve endings among 

 the secondary sense (hair) cells. (Redrawn from Johnson, 1917, J. Comp. Neurol., 28.) 

 (G) The lateral line sensory cord is shown growing posteriad within the epidermal 

 pocket of a 21 mm. embryo of Squaliis. (Redrawn from Johnson, 1917, J. Comp. Neurol., 

 28.) (H) Taste bud of human. (Redrawn from Neal and Rand, 1939, Chordate Anat- 

 omy, Philadelphia, Blakiston.) (I) Sagittal section through human nasal cavity de- 

 picting nasal conchae (turbinates) and various openings leading off from the lateral 

 wall of the nasal cavity. The olfactory area of the mucous membrane extends over the 

 superior concha and medially over the upper part of the nasal septum. Observe opening 

 of eustachian tube (tuba auditiva). 



