THE SPINAL NERVES. 67 



(entodermal) mechanism. This is its sole function in the 

 trunk. In its cranial portion it also supplies taste buds 

 and terminal buds (ectodermal organs). The communis 

 system as a whole corresponds with the fasciculus solitar- 

 ius of mammals, together with its associated stinictures 

 (sensory vagus nucleus, etc.), though the comparison is not 

 exact. Each of these tracts is related to a dorsal com- 

 missure, the commissura infima Halleri in the one case 

 and the fibres crossing in connection with the commissural 

 nucleus of Cajal in the other. 



Section 4. — The Spinal Nerves. 



I. — The Fourth Spinal Nerve. 



Our examination begins with the fourth spinal, which 

 may be regarded as the first typical trunk nerve. The 

 dorsal and ventral roots, of which the latter is the larger, 

 emerge by distinct but closely approximated foramina 

 through the base of the neural arch of the third free 

 vertebra. The roots, foramina and ganglion all lie in the 

 same transverse plane. The dorsal root {4. sp. d.) is 

 composed wholly of fine fibres, the ventral root chiefly or 

 wholly of coarse ones. From the ganglion are given off 

 two minute dorsal rami, a medium-sized medial ramus 

 and a large ventral ramus, besides the r. communicans 

 with the sympathetic trunk. 



The ventral ramus (r. v. 4) pursues the typical course. 

 The medial ramus {r. in. ^), like the ventral, contains 

 both sensory and motor elements. It passes laterad and 

 caudad, not in the intermuscular septum between the 

 dorsal and the lateral musculature, but through the lateral 

 part of the dorsal musculature, following for part of its 

 course one of the intermuscular bones, which it finally 



