The Nervous System. 39 



ponents, here better described as afferent and efferent. It connects the 

 central nervous system either with the actual viscera, or with visceral 

 structures in a general sense, the chief relations of the sympathetic 

 system being with the smooth muscle of visceral organs, or with that of 

 the bloodvessels in various parts of the body. 



The cranial nerves are comparable in some respects to the spinal 

 nerves, but in various ways are highly modified structures. The third, 

 fourth and sixth, respectively oculomotor, trochlear and abducent nerves, 

 which are distributed to the muscles of the eye, are considered as belong- 

 ing to the somatic motor division. The second, or optic nerve, and the 

 nervous portion, or retina of the eye, are specialized parts of the 

 central nervous system, arising embryonically as an outgrowth of the 

 latter. The nerves of the remaining special sense-organs, including the 

 first, or olfactory nerve and the eighth, or acoustic nerve, have been con- 

 sidered as parts of an extensive system of nerves, which, in lower aquatic 

 vertebrates, also contains representatives of the branchiomeric series, and 

 is distributed widely to sense-organs lying in the skin. 



Of the remaining cranial nerves the fifth, seventh, ninth and tenth are 

 branchiomeric. Although the connections of these nerves are not fully 

 considered in the dissection as here outlined, their chief characteristic as 

 branchiomeric structures may be indicated. The fifth, or trigeminal nerve, 

 the nerve of the mandibular arch, arises in two parts, one of which, the 

 portio major, is sensory, the other, the portio minor, motor. The portio 

 minor unites with the third or mandibular division of the portio major. 

 Thus, the terminal branches of all three divisions, ophthalmic, maxillary, 

 mandibular, are distributed as somatic sensory nerves to the skin of the 

 head, and, in addition, the mandibular nerve distributes visceral motor 

 branches to certain muscles (masticatory group, mylohyoid and digastric) 

 regarded as belonging to this, the first arch. A visceral sensory con- 

 nection with the mouth is considered to be formed by the lingual branch 

 of the mandibular nerve and by the palatine branches of the spheno- 

 palatine ganglion. Both are, however, connected with the central 

 nervous system through the seventh nerve, the former by the chorda 

 tympani, and the latter by the great superficial petrosal. 



The seventh, or facial nerve is the nerve of the second, or hyoid arch. 

 It is chiefly distributed as a visceral motor nerve to the cutaneous 

 muscles of the head. The ninth, or glossopharyngeal nerve, belonging to 

 the third arch, the tenth, or vagus, belonging to the fourth and succeeding 

 arches in lower forms, and the eleventh, or spinal accessory nerves, the 

 latter apparently related to the vagus as a motor portion, are distributed 

 as visceral motor nerves to the pharyngeal and laryngeal musculature, 

 and as visceral sensory nerves to various visceral organs, the ninth nerve 

 supplying the gustatory organs of the tongue. The spinal accessory, 

 moreover, has a characteristic distribution to the cleidomastoid, sterno- 

 mastoid and trapezius muscles of the side of the neck and shoulder. The 

 twelfth, or hypoglossal nerve has the relation of the ventral or motor 

 portion of a spinal nerve, and is distributed as a motor nerve to the 

 muscles of the tongue. 



