Johnston, Morphology of the Head. 1 8 1 



constituent elements. This outline differs from the one pre- 

 viously published chiefly in that the visual and olfactory organs 

 and the sympathetic system are assigned to definite places in 

 the scheme, a thing which was not attempted before. 



A. Somatic sensory division. 



1. General cutaneous subdivision. Consists of : 



free nerve endings in the skin, 



general cutaneous system of components, 



dorsal tracts of the cord, spinal V tract in the medulla oblongata, to- 

 gether with their accompanying nuclei : the dorsal horn, nucleus 

 funiculi, nucleus trigemini spinalis, acusticum and cerebellum, 



secondary tracts and centers : internal and external arcuate fibers form- 

 ing the tractus spino- and bulbo-tectalis (fillet), tectum opticum, 

 colliculus and other nuclei, 



tertiary tracts to motor nuclei and coordinating centers. 



2. Special cutaneous subdivision. Consists of : 



neuromasts (acustico-lateral sense organs), 



neuromast components, 



spinal VIII tract and nucleus, nucleus funiculi, acusticum, cerebellum, 



secondary tracts and centers and tertiary tracts as in i, 



the cochlea, its nerve and centers in higher vertebrates. 



3. Special sense organs belonging to the somatic sensory division. 



lateral eyes. Consist of: 



retina, which includes the equivalent of sensory ganglion, nerve com- 

 ponent, and primary brain center, 



optic tract and tectum opticum corresponding to the secondary tracts 

 and centers of the cutaneous subdivision. 



pineal eyes. (Compare Sec. 13 below). 



B. Splanchnic sensory division. 



1. General visceral subdivision. Consists of: 



free nerve endings in the mucosae, 



fasciculus communis system of components, 



Clarke's column or its equivalent, nucleus commissuralis Cajal, lobus 



vagi, and lobus facialis, 

 secondary vagus tract and its continuation in the cord, = the direct 



cerebellar tract in higher forms, 

 secondary vagus nucleus, ^ end nucleus of direct cerebellar tract in 



the vermis of higher forms, 

 tertiary tracts not well known ; part probably run to the thalamus or 



hypothalamus in lower vertebrates. 



2. Special visceral subdivision. Consists of: 



end buds (taste buds), 



components and central nuclei and tracts not yet distinguished from 

 those of the general visceral subdivision. 



3. Special sense organ belonging to the splanchnic sensory division. 



Consists of: olfactory epithelium and nerve, bulbus olfactorius, tractus 



