332 



Vertehrata. 



distinguishable from the pia mater in Fish, and in other 

 Vertebrata they are closely connected. A number of 

 pairs of nerves arise from the brain, in part sensoi-y 

 (tactile, olfactory, optic, and auditory), and in part motor ; 

 they are principally distributed to the head. Trom the 

 thalamencephalon and the optic lobes arise the optic 

 nerves, which are remarkable for crossing at their 

 point of origin ; the nerve of the right eye thus arises to 

 the left of the median Une, and crosses over. The 

 simplest case of such a crossing or chiasma occurs in 

 many of the Teleostei, where the two nerves simply 

 cross, without entering into any closer connection. In most 

 other Vertebrata, on the contrary, the optic nerves ex- 

 change some fibres at the chiasma, so that, whilst the 

 main portion of the nerve which originates on the right 

 side iTins to the left eye, a few of its fibres bend round 

 and join the other optic nerve, which, for its part, also 

 gives up some nerve fibres to its feUow. Of the other 

 cranial nerves, the olfactory arise from the olfactory lobes, 

 the others mostly from the ventral side of the medulla. 

 One of these, the vagus, is remarkable in that it not only 

 sends branches to the head, but also posteriorly, to supply, 

 for example, certain portions of the alimentary canal. The 

 spinal nerves nsnally leave the vertebral canal laterally, 

 one pair between every two successive vertebrae ; each nerve 

 originates from the cord in two roots, of which the dorsal 

 is furnished with a small ganglion close to its point of 

 origin, and contains exclusively sensory nerve fibres, whilst 

 the ventral root consists of motor fibres only. The nerves 

 supplying the limbs originate in a number of spinal nei-ves, 

 connected together into so-called plexuses (brachial plexus 

 and lumbar plexus, for the fore andhind limbs i-espectively). 

 A peculiar system is the so-called sympathetic ner- 

 vous system, the pi-incipal part of which is a pair of 

 stout nei-ve cords i-unning ventral to the vertebral column, 

 and only connected to the spinal cord and brain by small 

 connectives. The sympathetic nerves, which branch over 

 the alimentary canal and other viscera, are provided with 

 a great number of ganglia ; the movements of the parts 

 which they supply [e.g., the muscles of the alimentai-y 

 canal) are involuntai-y. 



Concerning tlie tactile and gustatory organs 

 of the Vertebrata, see the General Part, pp. 18 — 19. 



The olfactory organs are, in the Pish, 

 a pair of large pits on the surface of the head, 

 covered by an epithelium, in which are sensory 

 cells. In other Vertebrata, it is only in .early 

 embryonic stages that the olfactory organs are 

 such superficial elongate pits (Fig. 283) ; gradually 

 each pit becomes overgrown by the surrounding 

 parts, so that it forms a tube with an anterior and 

 a posterior orifice, of which the former opens 



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