As already mentioned, there are commonly two branches of communication between each of the spinal nerves and the sympa thetic in the higher animals. The one of these presents a white appearance, resem bling more or less the ordinary nerves of the cerebro-spinal system; the other has fre quently a more gray aspect, approaching in this respect the appearance of the sympa thetic nerves. Sometimes the white cord presents the appearance of being composed of a white and a grayer portion running to gether. As regards the minute structure of the rami cornmunicantes, the whiter portion consists entirely of tubular nerve-fibres, both of the coarser and finer varieties . there are also not unfrequently present fibres which appears to be intermediate in point of breadth. In general the broader variety of fibres ap pear to be more numerous than those which belong to the finer variety. According to Ki3Hiker, the relation between them is much the same in point of number as in the pos terior roots of the spinal nerves. The gray portion, as is stated by Todd and Bowman, contains a large proportion of fibres belonging to the gelatinous variety : in young animals it is often entirely composed of structures agreeing in character with the gelatinous fibre. In the full-grown animal also it often, when examined without addition of reagents, presents the appearance of being altogether composed of these fibres. Addition of dilute solution of soda, however, always brings into view a number of tubular nerve-fibres, which belong to the finer variety. The relation be tween the tubular fibres and those of the gelatinous kind as regards number, is much the same as in many of the branches of the sympathetic, especially in the smaller twigs distributed to the blood-vessels. Occasionally-, however, especially in the rabbit and cat, this portion is found to be almost exclusively composed of fine tubular nerve-fibres : the gelatinous fibres being present only in small numbers. In its appearance as seen by the naked eye, as well as in its microscopic structure, the grey portion of the raini com municantes agrees in character with the branches of the sympathetic, and would ap pear to be an offset from the same to the cerebro- spinal nerves. This is rendered more probable by the observation of Dr.
Deck, that the grey portions on leaving the ganglia send off small branches to the neigh bouring vessels, and are reduced in size before reaching the spinal nerves. Moreover, Kill liker has sometimes observed a small gan glion present upon them, which, on exami nation, was found to present the structure of the sympathetic ganglia, and which gave origin to the fibres with which it was con nected.
As regards the white portion of the rami communicantes, there can be no doubt that all the broad tubular fibres contained in it are fibres which are transmitted from the nerves of the cerebro-spinal system. This is proved by the fact that all the tubular fibres which are supposed to originate in the ganglia do not be long to the broader, but to the finer variety. In regard to the finer variety- of tubular nerve fibres occurring in this portion, inasmuch as similar fibres are present both in the anterior and posterior roots of the spinal nerves, they may be regarded either as fibres sent from the spinal cord to the sympathetic, or they may be fibres which are transmitted from the latter to the cerebro-spinal nerves. On tracing
the white portion of the rami communicantes backwards to the spinal nerve, it is found to apply itself to the latter generally in a direc tion more or less central. On attempting to separate the two by, rneans of needles, though several of the fibres break across, yet the di rection of these, as well as of the others, ap pears to be towards the centre. When the corresponding spinal nerve, along vvith its communicating branch, is dissected out, and examined, after addition of dilute solution of soda, with a power of 40 diameters, it is not difficult to observe, in the cat or other small animals, that the fibres composing the white portion run towards the centre. Many of them bend directly inwards to the cord, while others sink into the spinal nerve, more or less obliquely, still, however, in the direction of the centre. That the fibres in question are not to be regarded as fibres sent from the sym pathetic to the cerebro-spinal nerves is ren dered further probable by the fact that they can all be traced beyond the corresponding sympathetic ganglion into the cord above and below. Moreover KbHiker has traced them not only past the ganglia in the main chain of the sympathetic, but into the peripherical branches, and, in small animals, even through the ganglia occurring upon these latter He also finds that the fine fibres in question differ from those which arise in the sympathetic ganglia in presenting a darker contour and in being somewhat broader.
As regards the proportion between the fibres in the communicating branches which nnay be regarded as proceeding from the sym pathetic to the cerebro-spinal nerves and those which are sent by the latter to the sympa thetic, we have already seen that in the frog, according to the observations of Bidder and Volkmann, the former exceed the latter con • siderably. In the higher animals it would appear that the reverse is the case ; in the rabbit, according to KoHiker, by fitr the greater portion of the rand comnzunicantes run towards the centre. In man also, according to the same observer, much the greater number of the fibres contained in these branches run inwards towards the spinal column. I examined most of the rami communicantes in a fcetal calf about 24 feet in length, and have little hesitation in saying that in this animal the proportion of fibres which are directed towards the spinal cord greatly exceeds any that appear to run towards the periphery. At the point ofjunction with the spinal nerve the communicating branch, when examined with a power of 40 diameters, was seen to spread out somewhat, most of the fibres bent directly inwards towards the spinal cord, others passed into the nerve, either obliquely or at right angles to it, and then curved in wards towards the spinal cord. In some of the communicating branches, a few of the fibres were seen to join the nerve in the direc tion of the periphery. In the cat also, the fibres of many of the communicating branches were found to run exclusively in the direction of the spinal cord.