Many of the Rodentia are furnished with glands of this description, and they are situated on each side of the penis, immediately beneath the skin that covers the pubic region.

(2353). It is with the preputial glands that we must notice the still more elaborately-developed secreting organs of the Beaver, that furnish the drug called "castor." These organs, represented in the annexed figure (fig. 405), consist of large glandular pouches (g, h), that discharge their contents in the vicinity of the anal and preputial apertures; but of what importance the material thus abundantly secreted may be in the economy of the animals so provided, it is not easy to conjecture.

(2354). The secreting apparatus of the Musk Deer (Moschus moschi-ferus), which produces musk, is of analogous conformation. This is an oval pouch situated beneath the skin of the lower part of the belly: its walls are thin and apparently membranous; but the membrane that lines them is rugose and plicated. The orifice leading to this pouch is small, and opens in front of the prepuce.

* Cuvier, Lesons d'Anat. Comp. v. p. 252 et seq.

Sexual organs of male Beaver.

Fig. 405. Sexual organs of male Beaver.

(2355). Lastly, in connexion with these odoriferous glands we may mention the "temporal glands" of the Elephant, from the duct of which, situated on each side midway between the eye and the ear, there flows a viscid and fetid liquid; and likewise the "anal glands" met with in most Carnivora. The ducts of the glands last mentioned open near the margin of the anus; and in some genera, as the Skunk and the Polecat, the stench produced by the fluid poured from these sources is so intolerable as to become a most efficient defence against a foreign enemy.

(2356). We now come to consider the nervous system of the Mammalia, and are of course prepared to anticipate that in proportion as they surpass all other animals in intelligence, so will the encephalic masses assume a complexity and perfection of structure such as we have not hitherto witnessed in the whole series of the animal creation. Their senses likewise may be presumed to have attained the utmost delicacy of organization, in correspondence with the exalted attributes conferred upon this important class, and consequently to exhibit appendages and accessory parts adapting them most accurately to repeat to the sensorium impressions derived from without.

(2357). Abstruse as the study of the brain has been rendered by the chaotic assemblage of names applied by the earlier anatomists, in their bewilderment, to every definable portion of its substance, we have little doubt that, when the grand laws that have hitherto guided us in investigating the nervous system of the lower animals are had recourse to, the student will soon perceive how little difficulty there is in comparing even the brain of Man with the encephalon of the humbler Vertebrata examined in preceding pages, and thus tracing the progressive advances from simple to more complex organization.

(2358). The great lessons deduciblo from all that we have as yet seen relative to the essential organization of the nervous system are obvious enough. First, that all nerves, whether connected with sensation or the movements of the body, emanate from or are in communication with nervous masses called ganglia, which are, in fact, so many brains presiding over the functions attributable to the individual nerves. Secondly, that in the lower animals where these ganglia exist, they are comparatively small, and more or less completely detached from each other; but that in the Vertebrata such is the increased development of the central masses of the nervous system, that they coalesce, as it were, into one great organ called the cerebro-spinal axis; and thus that the encephalon and medulla spinalis are both made up of symmetrical pairs of ganglia appointed to different functions, but so intimately blended together that they are no longer distinguishable, except from the pairs of nerves with which they are connected.

(2359). Taking the above for axioms (and they are incontrovertible), let us proceed to analyse the cerebro-spinal axis of the Mammalia, and to compare it in simple terms with that of Birds, Reptiles, and Fishes already examined.

(2360). Commencing at the anterior extremity of the series, the first encephalic masses that present themselves are the "olfactory nerves" as the human anatomist has been pleased to call them, although in every one of the details connected with their anatomical structure and relations they confessedly differ from every nerve in the body. They are, in truth, not nerves at all, but brains - the ganglia or brains of smell, from which the olfactory nerves, properly so called, invariably emanate. In Fishes (§ 1802) they were found to equal or even to surpass in size the hemispheres themselves. In Reptiles and Birds they became gradually concealed by the development of the hemispherical masses; and in the Mammalia such is their diminutive appearance, when compared with the cerebrum, that they are scarcely recognized as elements of the encephalon at all.

(2361). In all the oviparous Vertebrata the nerves of smell were two simple cords, one derived from each of the olfactory ganglia, from which they proceeded through osseous canals to the nose. But in the Mam-mifers these nerves are extremely numerous, in proportion to the extent of the surface to be supplied, and escape from the skull through the cranial plate of the ethmoid bone, which, from the number of apertures that it offers for their passage into the nose, richly merits the name of "cribriform," more especially in the carnivorous quadrupeds possessed of the most acute smell.

Olfactory apparatus of the Lion.

Fig. 406. Olfactory apparatus of the Lion.