This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2067). The upper larynx, or rima glottidis, is in Birds but of secondary importance in the production of vocal sounds: it is a simple fissure bounded by two osseous pieces (fig. 361, a, b, f), corresponding with the arytenoid cartilages of Mammalia; these, however, in the Bird are not connected with chordce vocales, but simply, as they are separated or approximated, open or close the fissure of the glottis. When, therefore, we compare the framework of this organ with the cartilaginous pieces found in the larynx of Mammalia, considerable difference is perceptible, insomuch that it is not easy positively to recognize the analogous portions, more especially as in the Bird the cartilages are more or less completely ossified. If the broad anterior plate (fig. 361, 6) be considered as the thyroid cartilage, we must suppose the cricoid to be represented by three distinct ossicles, two of which (cc) are lateral, while the third or central portion (e) supports the arytenoid bones (ff), which are moveably articulated with its anterior margin. The arytenoid bones themselves are of an elongated form, and each presents a long process (g g) for the insertion of the muscles that act upon them. These arytenoid bones are moved by two pairs of muscles - the superficial pair (thyro-arytenoidei) (fig. 362, b) serving to pull asunder, while the more deeply-seated (constrictores glottidis) (fig. 362, a) bring together the lips of the glottis.
(2068). It is the lower larynx, situated at the opposite extremity of the trachea, at the point where that tube gives off the bronchi, that the real vocal apparatus of birds is situated; and in the more perfect Singing Birds a very important set of muscles is appropriated to perform those delicate movements that regulate the condition of the air-passages at this part, and thus give rise to all the varieties of tone of which the voice is capable.
Fig. 361. Cartilages of the superior larynx of a bird.
(2069). In the Insessorial Birds, by far the most accomplished songsters, five pairs of muscles are connected with the inferior larynx, and so disposed as to influence both the diameter and length of the bronchial tubes (fig. 360, a, b, n, o, z, s, h.) In the Parrots, three pairs only are met with*; some of the Natatores have two; other Natatorial birds, as well as the Rasores and Gral-latores, only one; and in a few, as the King of the Vultures and the Condor, the vocal muscles are quite deficient.
(2070). Not only is the respiration of these highly-gifted Vertebrata thus abundantly provided for, but, as an immediate consequence of the necessity for supplying the system with pure and highly-oxygenized blood, the heart, hitherto but imperfectly divided, becomes now separated into two distinct sets of cavities, each composed of an auricle and of a strong ventricular chamber. The right side of the heart receives the vitiated blood from all parts of the system, which is poured into the corresponding auricle by three large veins, viz. one inferior and two superior venoe cavae. The contraction of this auricle drives the blood into the right ventricle, - the auriculo-ventricular opening being guarded by a broad fleshy valve, formed by the muscular substance of the heart itself; and hence the venous blood is forced through all the ramifications of the pulmonary arteries.
(2071). The aerated blood is then returned from the lungs by two veins, which pour it into the left auricle; and the left ventricle, now entirely appropriated to the systemic circulation, diffuses it through the body: thus, all mixture of the venous and arterial fluids being prevented, the system is supplied by the left side of the heart with pure and highly-vitalized blood.
(2072). In the nervous system of Birds there is a very perceptible improvement when compared with that of Reptiles, more especially in the increased proportional development of the cerebral hemispheres: still, however, there are no convolutions seen upon the surface of the cerebrum, neither are those extensive communications between the lateral halves as yet developed which in the higher Mammalia assume such size and importance; the corpus callosum and fornix are both wanting, a simple commissure being still sufficient. Neither has the cerebellum in these animals assumed its complete development, presenting only the central portion; so that the pons Varolii, or the great commissure which in Man unites the lateral cerebellic lobes, is of course deficient. The olfactory and optic lobes are even here recognizable as distinct elements of the cerebral mass, and the origins of the nerves strictly conform to the arrangement already described in the brain of Reptiles. The rest of the cerebro-spinal axis presents no peculiarity worthy of special notice; and the general distribution of the cerebral and spinal nerves is so similar in all the Vertebrata, that it would be useless again to describe them in this place.
Fig. 362. Muscles of the superior larynx of a bird.
* Vide Yarrell on the Organs of Voice in Birds (Linn. Trans, vol. xvi.).
(2073). The sympathetic system in Birds is well developed, and its arrangement differs in no essential particular from what is seen in the human body: the situation of the cervical ganglia is, however, peculiar, inasmuch as they are lodged in the bony canal formed by the transverse processes of the vertebrae of the neck for the reception of the vertebral artery, and are thus securely protected, in spite of the unusual length and slenderness which the neck not unfrequently exhibits.
(2074). But if in the general arrangement of the nervous system of the feathered races there is little to arrest our notice, we shall find in the construction of the organs of their senses many circumstances of considerable interest to the physiological reader; and consequently these will require a more extended description.