IV. OF RANA PIPIENS. 41 



If we apply the analogies of the spinal cliord and vertebral column to the 

 cranium and its nerves, we ought to base our determinations on the repetitions of 

 true spinal nerves and of the true vertebral elements. If the theory be true which 

 reduces the cranial nerves (exclusive of the special sense nerves) to three, namely, 

 the trigeminus, vagus, and hypoglossus, tlien we ought, a priori, to detect at 

 least three vertebral segments. This conclusion agrees perfectly well with tlie deter- 

 mination from osteology. For the larger part of modern anatomists admit at 

 least three vertebra?, though some admit more, but are not precisely agreed as to 

 the exact number of elements which enter into the composition of each. These 

 vertebrce may be designated as follows : 1st, the occipital, of which the basilar 

 bone is the body; 2d, the parietal, of wliich the posterior sphenoid is the body; 

 3d, the frontal, of which the anterior sphenoid is the body. Professor Owen ad- 

 mits a fourth, of which the vomer is the body, and as this vertebra is associated 

 with the organs of smell, he designates it as the rhinencephalic vertebra. It is 

 through or between these three vertebrae enumerated above, that the trigeminus, 

 vagus, and hypoglossus have their exit from the cranial cavity, and it is likewise 

 through or between these same vertebrae that the special sense nerves make their 

 escape. If the number of pairs of nerves of both kinds is to regulate the num- 

 ber of vertebrae, then, instead of three pairs, we must admit six for the cranium 

 alone, leaving wholly out of view the face. But if the special sense nerves, for 

 reasons already stated, can be rejected as indications of vertebrEC, the cranio-spinal 

 nerves will give us just the number which accords with osteology. In this con- 

 clusion we are supported by both osteology and neurology. Johannes Muller 

 admits the existence of three vertebrae, and argues from them the number of pairs 

 of cranio-spinal nerves. " According to my view, there are three vertebral nerves 

 of the head, just as there are three cranial vertebrae. The first is the fifth or tri- 

 geminus, the second is the vagus with the glosso-pharyngeal and accessory, and the 

 third is the hypoglossus." 



The three-vertebra theory given above relates simply to the bones constituting 

 the walls of the cranial cavity which include the brain ; no account is taken of 

 the jaws and otlier bones of the face, nor of the os hyoides. Professor Owen, in 

 his system, regards these, with the scapular arch, as forming a series of four in- 

 ferior or " hai-mal " arches to the four cranial vertebrae, with their " neural arches," 

 that they in fact arc serial repetitions of ribs. 



It would be foreign to the purpose of this paper to discuss the grounds on which 

 these conclusions rest. Another hypothesis seems to us worthy of consideration, 

 but which can only be stated in general terms, as follows. The teeth in the early 

 embryonic conditions are developed from, and are dependences of, the mucous 

 membrane of the mouth ; in many Fishes these conditions are permanent ; in Kays, 

 Sharks, and other cartilaginous Fishes, the jaws or other bones which support 

 teeth are equally developed in the internal integument or mucous membrane, 

 and are never closely connected with the cranium except by ligament ; the hyoid 

 apparatus is likewise developed in the walls of the alimentary canal. If to these 

 facts we add another, namely, that primarily the mouth and nostrils form a single 



