998 



PISCES. 



sends a branch across to join its fellow on the 

 right side in the splanchnic nerve. This forms 

 a ganglionic enlargement on the mesenteric 

 artery, and, after communicating with the right 

 trunk of the par vagum, terminates on the intes- 

 tines and other viscera. On each side of the 

 aorta the prolongation of the sympathetic is 

 continued down to the tail, giving filaments to 

 the lateral branches proceeding from the aorta, 

 and communicating with the spinal nerves. 

 Near the anus filaments are sent off, which 

 unite and accompany the spermatic artery to 

 the ovaries. 



According to the united testimony of Costa, 

 Rathke, and Goodsir, no vestige of a brain 

 or encephalic enlargement of the medulla 

 spinalis is visible in Branchiostoma, a fact of 

 extreme interest to the physiologist. In these 

 extraordinary Fishes, the spinal cord, as de- 

 scribed by the last mentioned gentleman, 

 stretches along the whole length of the spine, 

 is acuminated at both ends, and exhibits not 

 the slightest trace of cerebral developement. 

 It is most developed in its middle third, where 

 it has the form of a riband, the thickness of 

 which is about one-fourth or one-fifth of its 

 breadth ; and along this portion also it pre- 

 sents on its upper surface a broad but shallow 

 groove. The other two-thirds are not so flat, 

 and are not grooved above. They taper off 

 gradually, the one towards the anterior, the 

 other towards the posterior end of the Fish. 



From fifty-five to sixty nerves pass off from 

 each side of the cord ; but as the anterior and 

 posterior vertebra are very minute and run into 

 one another, and as the spinal cord itself almost 

 disappears at the two extremities, it is impos- 

 sible to ascertain the exact number either of 

 vertebrae or spinal nerves. These nerves, Mr. 

 Goodsir assures us, are not connected to the 

 spinal marrow by double roots, but are inserted 

 into its edges in the form of simple cords. 



The nerves pass out of the intervertebral for- 

 amina of the membranous spinal canal, divide 

 into two sets of branches, one set (dorsal 

 branches) running up between the dorsal mus- 

 cular bundles; the others (ventral branches) 

 run obliquely downwards and backwards on 

 the surface of the fibrous sheath of the vertebral 

 column, and are distributed to the muscles of 

 the ventral region. 



When an entire animal is examined by trans- 

 mitted light and a sufficient magnifying power, 

 the anterior extremity of the spinal cord is ob- 

 served, as before mentioned, to terminate in a 

 minute filament above the anterior extremity of 

 the vertebral column. The first pair of nerves 

 is excessively minute, and passes to the parts 

 around the mouth. The second pair is consi- 

 derably larger; it sends a considerable branch, 

 corresponding to the dorsal branches of the 

 other nerves, passes upwards and backwards 

 along the anterior edge of the first dorsal mus- 

 cular bundle. This branch joins the dorsal 

 branches of the third and of a considerable 

 number of the succeeding pairs of nerves, at 

 last becoming too minute to be traced further. 



After sending off this dorsal branch the se- 

 cond pair passes downwards and backwards on 



each edge above the hyoid apparatus, and joins 

 all the ventral branches of the other spinal 

 nerves in succession, as its dorsal branch did 

 along the back. This ventral branch of the 

 second pair is very conspicuous and may be 

 traced beyond the anus, but is lost sight of 

 near the extremity of the tail ; it evidently cor- 

 responds with the nerve represented in fig. 531, 

 1, as the dorsal communicating branch does 

 with the nerve marked 4 in the same figure. 



Sense of smell. In the structure of their 

 olfactory apparatus Fishes present a remark- 

 able difference from all other vertebrate ani- 

 mals; their nostrils are in fact quite uncon- 

 nected with the respiratory passages, consisting 

 of mere sacculi, into which the surrounding 

 water obtains free access, which are lined 

 with a pituitary membrane folded into regular 

 plicae, so as to offer an extensive surface for 

 contact. Their usual situation is towards the 

 fore part of the face, where they are supported 

 by the vomer, the maxillary, and the intermax- 

 illary bones, the first suborbital bounding their 

 lower margin, while above they are arched 

 over by a bone distinguished by Cuvier as the 

 nasal. 



The openings of the nostrils are of a round, 

 oval, or oblong shape; they are situated either 

 at the end of the muzzle or upon its sides, or 

 upon its upper surface, or sometimes even 

 beneath, as in the Rays and Sharks, where 

 they are found near the angles of the mouth. 

 In the Lamprey they are placed quite at the 

 summit of the head, and open by a common 

 orifice ; but in the greater number of Fishes, 

 perhaps in all the osseous races, each olfactory 

 sacculus presents two orifices, one in front, the 

 other behind, which are sometimes sufficiently 

 remote from each other, but both orifices open 

 into the same cavity. 



The anterior orifice sometimes has its edges 

 tubular, as in the Eel, and sometimes this 

 tubular edge is prolonged, as in the Lote and 

 some of the Silundae, into a tentacle of more 

 or less considerable length : at other times these 

 tubular prolongations are wanting, as in the 

 Scombridse, in which family, moreover, the 

 posterior nostrils are but vertical slits. 



The nostrils of the Lophius offer a remark- 

 able peculiarity, each being supported upon a 

 little pedicle so as to resemble a mushroom, the 

 expansion of the mushroom containing the 

 olfactory cavity, which, as usual, communicates 

 with the exterior by two little orifices. 



In some rare instances the posterior aperture 

 of the olfactory sacculus is situated beneath 

 the lip, a circumstance which is more especially 

 remarkable in some foreign Congers, and ex- 

 hibits a remarkable approximation to what is 

 met with in the amphibious Proteus and Siren. 



The disposition of the pituitary membrane 

 that lines the nasal sacculus is very simple; 

 where the shape of the olfactory cavity is round, 

 the folds of the membrane which lines it are dis- 

 posed like the radii of a circle (Jig. 529); but if 

 the nasal fossae are oblong or elongated, they 

 are arranged along the two sides of an axis in 

 very regular folds, resembling in their arrange- 

 ment the barbs of a feather. In the number 



