THE MOKPHOLOGY OF THE CEEEBKAL NEKVES. 



795 



Ramus Thyreohyoideus. The nerve to the thyreo-hyoid muscle is a small 

 Branch which arises from the hypoglossal nerve before it passes medial to the mylo- 

 lyoid muscle. It descends behind the greater cornu of the hyoid bone to reach 

 ;he muscle. When traced backwards this nerve is found associated with the loop 

 setween the first and second cervical nerves. 



Rami Linguales. The lingual branches of the hypoglossal nerve are distributed 

 bo the hyoglossus, genio-hyoid, and genioglossus, and to all the intrinsic muscles 

 )f the tongue. The nerve to the genio-hyoid is said to be derived from the loop 

 oetween the first and second cervical nerves. It is not known if these two cervical 

 aerves are implicated in the innervation of the proper muscles of the tongue, but 

 .t appears certain that the muscles named the genio-hyoid, thyreo-hyoid, sterno- 

 tiyoid, omo-hyoid, and sterno-thyreoid are not supplied by the hypoglossal, but 

 Dnly by cervical nerves, the genio-hyoid and thyreo-hyoid by the first two, the 

 Dther muscles by the first three cervical nerves. 



THE MORPHOLOGY OF THE CEREBRAL NERVES. 



The head and face, possibly the oldest, and from every point of view the most fundamental 

 md important portion of the body fabric, present in some respects a more conservative type of 



LATERAL AREA 



MEDIAL AREA 



I BASAL 

 I LAMINA 



f ANTERIOR ROOT 

 'POSTERIOR ROOT 



A 



ix.x.xr 



FIG. 665. COMPARISON OF ORIGINS OF NERVE ROOTS FROM SPINAL MEDULLA AND HIND-BRAIN (after His). 



A. Spinal medulla ; B. Hind-brain. 



structure, and in other aspects have been subject to more profound alterations than other parts 

 of the body. Segmentation is characteristic of the trunk, pervading bones, muscles, vessels, and 

 nerves. An absence of true segmentation is characteristic of the head region omitting for the 

 moment the cerebral nerves. The head is characterised by the possession of an unsegmented 

 tubular nervous system, enclosed in a bony capsule not obviously segmental, with which the 

 capsules of the sense-organs become united. The pre-oral and post-oral visceral arches and clefts 

 are not truly segmental like the costal arches of the trunk. The branchial clefts are said to be 

 inter-segmental ; and their muscles (associated with the myoblast surrounding the developing 

 heart) are described as visceral, and not myotomic, so that the branchial vessels and nerves 

 (similarly) are not to be regarded as comparable to the segmental vessels and nerves of the trunk. 

 The truly segmental structures present are certain persistent myotomes or muscle plates, which 

 give rise to muscles innervated by the third, fourth, sixth, and twelfth cerebral nerves. 



Another difficulty in the morphology of the head arises from the absence of body cavity, and 

 the consequent difficulty of differentiating the somatic and splanchnic mesoderm, and the somatic 

 and splanchnic distribution of a given nerve. 



Under these circumstances there is little help to be derived from head structures other than 

 the nerves themselves in seeking a solution of the question of the morphological relations of the 

 cerebral nerves. The spinal nerves are, generally speaking, all alike. The cerebral nerves, on 

 the other hand, are all different. Scarcely any two nerves are alike ; and no single cerebral nerve 

 possesses in itself all the characteristic features of a spinal nerve. As seen in the account of its 

 development (p. 504), the cranial nervous system possesses a series of dorsal ganglia, comparable 

 n position and development to the spinal ganglia with which afferent nerves are associated ; 

 and the efferent roots are developed in the same way, and occupy somewhat the same position 



